Fix crash in struct-with-sig-2.exp with debug-names target board

When I run the struct-with-sig-2.exp test with the .debug_names-using
target board, I see a gdb crash.  This happens because the reader
throws an exception without calling finalize_all_units.  This causes
an assertion failure later because the number of CUs and TUs doesn't
match.

The fix is to clear 'all_units' on failure.

Approved-By: Tom de Vries <tdevries@suse.de>

gdb/dwarf2/read.c

@@ -5193,6 +5193,8 @@ create_all_units (dwarf2_per_objfile *per_objfile)
 
       if (!dwz->types.empty ())
 	{
+	  per_objfile->per_bfd->all_units.clear ();
+
 	  /* See enhancement PR symtab/30838.  */
 	  error (_("Dwarf Error: .debug_types section not supported in dwz file"));
 	}