bfd_get_symbol_leading_char vs. ""

bfd_get_symbol_leading_char vs. ""

Some places matching the first char of a string against
bfd_get_symbol_leading_char, which may be zero, didn't check for the
string being "".  This patch adds the check to stop accesses past the
end of the string and potential buffer overruns.
The dlltool one was found by oss-fuzz quite a while ago.

bfd/
	* cofflink.c (_bfd_coff_link_input_bfd): Ensure a zero
	bfd_get_symbol_leading_char doesn't lead to accessing past the
	zero string terminator.
	* linker.c (bfd_wrapped_link_hash_lookup): Likewise.
	(unwrap_hash_lookup): Likewise.
binutils/
	* dlltool.c (scan_filtered_symbols): Ensure a zero
	bfd_get_symbol_leading_char doesn't lead to accessing past the
	zero string terminator.