LPC2148 UART interrupt

I am using LPC2148 with SIM800L on UART1 using interrupt. When I use memset(buff, 0 , sizeof(buff)); in line "75" at first the program works fine but when while(1) loop runs again it prints error over and over but when I comment the memset(buff, 0, sizeof(buff)); it works fine.

So why it is not receiving the data on using memset function?

code

#include <lpc214x.h>
#include <stdio.h>
#include <string.h>


__irq void uart1_interrupt(void);

void init_uart1(void);						//////initialize uart1
void uart1_write(char data);			//////send char on uart1
void uart1_send_string(char *str);//////send string on uart1

void Uart0_string(char *str);			//////send string on uart0
void U0write(char data);					//////send char on uart0
void init_uart0(void);						//////initialize uart0

void pll_init(void);
void timer_init(void);
void delay_ms(unsigned int ms);

//void clear_data();								/////clear buffer

char buff[160];
//char sms_receive[60];
int i, a = 0;

int main(void)
{
	pll_init();
	timer_init();
	init_uart0();
	//init_uart1();

	
	VICVectAddr0 = (unsigned) uart1_interrupt;                /////attach interrupt routine
	VICVectCntl0 = (1<<5)|7; 																	////enable uart irq slot for uart1
	VICIntEnable |= (1<<7); 																	////enable uart1 interrupt 
	VICIntSelect = 0x00000000;
	init_uart1();
	
	Uart0_string("gsm test\r\n");
	
	
	delay_ms(3000);
	
	while(1)
	{
		uart1_send_string("AT\r\n");
		delay_ms(400);
		if(strstr(buff,"OK"))
		{
			Uart0_string(buff);
		}
		
		else
			{
				Uart0_string("ERROR ");
			}
			
			delay_ms(3000);
			
			uart1_send_string("AT+CNMI=?\r\n");
			delay_ms(400);
			if(strstr(buff,"+CNMI"))
			{
				Uart0_string(buff);
			}
			
			else
				{
					Uart0_string("ERROR ");
				}
				
				delay_ms(3000);
				//clear_data();
		    memset(buff, 0 , sizeof(buff));
	}
}


/*void clear_data()
{
	for(i = 0; i<=sizeof(buff); i++)
	{
		buff[i] = 0;
	}
}*/

__irq void uart1_interrupt(void)
{
	buff[a] = U1RBR;
	a++;
	VICVectAddr = 0x00;
}

void init_uart1(void)
{
	PINSEL0 = PINSEL0 | 0x00050000;    ////UART1 pin selection
	
	U1LCR = 0x83; 			 ////8 bit, no parity, 1 stop bit, DLAB - 1
	U1DLL = 0x56;				 ////86 in decimal
	U1DLM = 0x00;
	U1FDR = 0xF2;
	
	U1LCR &= 0x0f;			/////setting DLAB bit to 0
	
	U1IER = 0x01;				////enable interrupt
}

void uart1_write(char data)
{
	U1IER = 0x00;
	while(!(U1LSR &(1<<5)));		////U1LSR contains THRE status bit, wait while THR is empty
	U1THR = data;
	U1IER = 0x01;
}

void uart1_send_string(char *str)
{
	U1IER = 0x00;
	while(*str != '\0')
	{
		uart1_write(*str++);
	}
	U1IER = 0x01;
}


/*char uart1_receive()
{
	while(!(U1LSR & (1<<0)));
	return U1RBR;
}*/

void init_uart0()
{
	PINSEL0 = 0x00000005;   ////selects TxD P0.0 and RxD P0.1
	
	U0LCR = 0x83;			////8 bit, no parity 1 stop bit, DLAB = 1
	U0DLL = 0x56;     //// 86 in decimal
	U0DLM = 0x00;
	U0FDR = 0xF2;			////MULVAL and DIVADDVAL values
	
	U0LCR &= 0x0F;
}

void U0write(char data)
{
	while(!(U0LSR & (1<<5)));  ///////wait till THR is not empty
	U0THR = data;							 /////// write data to Transmit in U0THR
	
}

void Uart0_string(char *str)
{
	while(*str != '\0')
	{
		U0write(*str++);
	}
}

void pll_init(void)
{
	  PLL0CON = 0x01;        /////enable PLL////
		PLL0CFG = 0x24; 			 /////configure PLL/////
		PLL0FEED = 0xAA;			 //////feed sequence/////
		PLL0FEED = 0x55;			 //////feed sequence/////
		
		while(!(PLL0STAT & 0x00000400)); ///is locked////
			
		PLL0CON = 0x03;		////connect PLL////
		PLL0FEED = 0xAA;
		PLL0FEED = 0x55;
		
		//VPBDIV = 0x01; /////// PCLK = CCLK = 60MHz
		VPBDIV = 0x00;	 /////// PCLK = 60/4 = 15MHz
}

void timer_init(void)
{
	//T0TCR = 0x00;
		T0CTCR = 0x0;    	////selection of timer mode with every rising pclk edge
		T0PR = 14999;			////prescale register value will generate 1 ms at 60MHz
		T0TCR = 0x02;     ////reset the timer counter and prescale couter
}
	
void delay_ms(unsigned int ms)
{
		//T0TC = 0x00000000;
		T0TCR = 0x02;     /////reset timer. Timer should be reset
		T0TCR = 0x01;     /////enable timer to generate delay
		T0MR0 = ms;
		while(T0TC < T0MR0); /////wait until Timer counter register reaches desired delay
		T0TCR = 0x00;     /////disable timer
	
}

output when using "memset(buff, 0 , sizeof(buff))"

gsm test
AT
OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
ERROR ERROR ERROR ERROR

output when not using memset funciton

gsm test
AT
OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK
AT+CNMI=?
+CNMI: (0-3),(0-3),(0,2),(0,1),(0,1)

OK
AT
OK