Fix a segmentation fault triggered by disassembling an EFi file with source included.

Fix a segmentation fault triggered by disassembling an EFi file with source included.

	* dwarf2.c (_bfd_dwarf2_find_symbol_bias): Check for a NULL symbol
	table pointer.
	* coffgen.c (coff_find_nearest_line_with_names): Do not call
	_bfd_dwarf2_find_symbol_bias if there is no symbol table available.

https://bugzilla.redhat.com/show_bug.cgi?id=1685727