py/makeqstrdefs.py: Fix beaviour when scanning non-C preprocessed files.

When process_file() is passed a preprocessed C++ file for instance it won't
find any lines containing .c files and the last_fname variable remains
None, so handle that gracefully.

py/makeqstrdefs.py

@@ -58,7 +58,8 @@ def process_file(f):
             elif args.mode == _MODE_COMPRESS:
                 output.append(match)
 
-    write_out(last_fname, output)
+    if last_fname:
+        write_out(last_fname, output)
     return ""