Fix stack buffer overflows when parsing corrupt ihex files.

Fix stack buffer overflows when parsing corrupt ihex files.

	PR binutils/18750
	* ihex.c (ihex_scan): Fixes incorrect escape sequence in error message
	and stack overflow when char is signed and \200-\376 was in place of hex
	digit; also fixes \377 was handled as EOF instead of "incorrect character".
	(ihex_read_section): Changed for consistency.
	(ihex_bad_byte): Prevent (now impossible to trigger) stack
	overflow and incorrect escape sequence handling.
	* srec.c (srec_bad_byte): Likewise.

	* readelf.c (process_mips_specific): Fix incorrect escape
	sequence handling.