This patch fixes a flaw in the SREC parser which could cause a stack overflow

This patch fixes a flaw in the SREC parser which could cause a stack overflow
and potential secuiryt breach.

	PR binutils/17510
	* srec.c (srec_bad_byte): Increase size of buf to allow for
	negative values.
	(srec_scan): Use an unsigned char buffer to hold header bytes.