asan: segfault in coff_mangle_symbols

asan: segfault in coff_mangle_symbols

The testcase managed to trigger creation of a wild pointer in
coff_slurp_symbol_table.  Stop that happening, and fix an unrelated
problem I happened to see in bfd_coff_get_syment.

	* coff-bfd.c (bfd_coff_get_syment): Clear fix_value after
	converting n_value from a pointer to an index.
	* coffcode.h (coff_slurp_symbol_table <C_BSTAT>): Sanity check
	symbol value before converting to a pointer.