* srec.c: Better error checking, partly from Peter Schauer.

* srec.c:  Better error checking, partly from Peter Schauer.
(srec_object_p):  Avoid assuming any file that starts with 'S' is
an S-record file!  (Check for 3 uppercase-hex digits after it.)
(ISHEX):  Define new macro for checking supposedly hex characters.
(pass_over):  Treat as EOF any S-record line that:
    *  doesn't have hex in either size character
    *  produces a size larger than 0x7F
    *  has an unrecognized type number