bfd/dwarf2.c: allow use of DWARF5 directory entry 0

bfd/dwarf2.c: allow use of DWARF5 directory entry 0

I think the test for table->files[file].dir being non-zero is wrong
for DWARF5 where index zero is allowed and is the current directory of
the compilation.  Most times this will be covered by the use of
table->comp_dir (from DW_AT_comp_dir) in concat_filename but the point
of putting the current dir in .debug_line was so the section could
stand alone without .debug_info.

Also, there is no need to check for table->dirs non-NULL, the
table->num_dirs test is sufficient.

	* dwarf2.c (concat_filename): Correct and simplify tests of
	directory index.