drive_new() returns null without setting an error when it provided
help.  add_init_drive() assumes null means failure, and crashes trying
to report a null error.

Fixes: c4f26c9f
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>