qemu-img: Check create_opts before image amendment

The image options which can be amended are described by the .create_opts
field for every driver. This field must therefore be non-NULL so that
anything can be amended in the first place. Check that this holds true
before going into qemu_opts_create() (because if .create_opts is NULL,
the create_opts pointer in img_amend() will be NULL after
qemu_opts_append()).

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -2986,6 +2986,13 @@ static int img_amend(int argc, char **argv)
         goto out;
     }
 
+    if (!bs->drv->create_opts) {
+        error_report("Format driver '%s' does not support any options to amend",
+                     fmt);
+        ret = -1;
+        goto out;
+    }
+
     create_opts = qemu_opts_append(create_opts, bs->drv->create_opts);
     opts = qemu_opts_create(create_opts, NULL, 0, &error_abort);
     if (options && qemu_opts_do_parse(opts, options, NULL)) {