Fix more potential seg-faults in gprof.

Fix more potential seg-faults in gprof.

	PR gprof/20499
	* corefile.c (num_of_syms_in): Return an unsigned int.
	(core_create_syms_from): Catch a possible integer overflow
	computing the argument to xmalloc.  Also allow for the possibility
	that an integer overflow in num_of_syms_in means that less space
	has been allocated than expected.