PR28862, heap-buffer-overflow in parse_stab_string

I have no info on the format of a "SUNPRO C++ Namespace" stab, so am
relying on the previous code being correct in parsing these stabs.
Just don't allow NULs anywhere in the stab.

	PR 28862
	* stabs.c (parse_stab_string): Don't overrun buffer when parsing
	'Y' stab.