qemu-img: Check create_opts before image creation

If a driver supports image creation, it needs to set the .create_opts
field. We can use that to make sure .create_opts for both drivers
involved is not NULL for the target image in qemu-img convert, which is
important so that the create_opts pointer in img_convert() is not NULL
after the qemu_opts_append() calls and when going into
qemu_opts_create().

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>
(cherry picked from commit f75613c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qemu-img.c

@@ -1378,6 +1378,20 @@ static int img_convert(int argc, char **argv)
         goto out;
     }
 
+    if (!drv->create_opts) {
+        error_report("Format driver '%s' does not support image creation",
+                     drv->format_name);
+        ret = -1;
+        goto out;
+    }
+
+    if (!proto_drv->create_opts) {
+        error_report("Protocol driver '%s' does not support image creation",
+                     proto_drv->format_name);
+        ret = -1;
+        goto out;
+    }
+
     create_opts = qemu_opts_append(create_opts, drv->create_opts);
     create_opts = qemu_opts_append(create_opts, proto_drv->create_opts);