qemu-img: Plug memory leak in convert command

Introduced in commit 661a0f7.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1455,7 +1455,7 @@ static int img_convert(int argc, char **argv)
     ret = bdrv_parse_cache_flags(cache, &flags);
     if (ret < 0) {
         error_report("Invalid cache option: %s", cache);
-        return -1;
+        goto out;
     }
 
     out_bs = bdrv_new_open("target", out_filename, out_fmt, flags, true, quiet);