argparse: Fix argv index overrun when argc = 0

If argc is 0, the index returned by mm_arg_parse may be 1 while it
should ave been 0. This is fixed ensuring that index is never bigger
than argc.

Change-Id: I0cfb7aa29eedc11506fb844234512702fdfdfec8

src/argparse.c

@@ -1354,7 +1354,7 @@ int mm_arg_parse(const struct mm_arg_parser* parser, int argc, char* argv[])
 			return early_stop_parsing(parser, r);
 	}
 
-	return index;
+	return index > argc ? argc : index;
 }
 
 /**