qemu-img: Fix crash in amend invocation

Example:
$ ./qemu-img create -f qcow2 /tmp/t.qcow2 64M
$ ./qemu-img amend -f qcow2 -o backing_file=/tmp/t.qcow2, -o help \
    /tmp/t.qcow2

This should not crash. This actually is tested by iotest 082, but not
caught due to the segmentation fault being silent (which is something
that needs to be fixed, too).

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
