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.

Author: amodra

bfd/dwarf2.c

@@ -2044,19 +2044,16 @@ concat_filename (struct line_info_table *table, unsigned int file)
       char *subdir_name = NULL;
       char *name;
       size_t len;
-
-      if (table->files[file].dir
-	  /* PR 17512: file: 0317e960.  */
-	  && table->files[file].dir
-	  <= (table->use_dir_and_file_0 ? table->num_dirs - 1 : table->num_dirs)
-	  /* PR 17512: file: 7f3d2e4b.  */
-	  && table->dirs != NULL)
-	{
-	  if (table->use_dir_and_file_0)
-	    subdir_name = table->dirs[table->files[file].dir];
-	  else
-	    subdir_name = table->dirs[table->files[file].dir - 1];
-	}
+      unsigned int dir = table->files[file].dir;
+
+      if (!table->use_dir_and_file_0)
+	--dir;
+      /* Wrapping from 0 to -1u above gives the intended result with
+	 the test below of leaving subdir_name NULL for pre-DWARF5 dir
+	 of 0.  */
+      /* PR 17512: file: 0317e960, file: 7f3d2e4b.  */
+      if (dir < table->num_dirs)
+	subdir_name = table->dirs[dir];
 
       if (!subdir_name || !IS_ABSOLUTE_PATH (subdir_name))
 	dir_name = table->comp_dir;