Fix unreachable use-before-assign in test_util_join_win_cmdline

Apparently there is a compiler that believes this is something to
warn about.

src/test/test_util.c

@@ -2713,7 +2713,7 @@ test_util_join_win_cmdline(void *ptr)
   };
 
   int i;
-  char *joined_argv;
+  char *joined_argv = NULL;
 
   (void)ptr;