Fix monitor command (screendump) (Stefan Weil)

starting with r6839, the monitor command 'screendump'
raises a nullpointer memory access which crashs Qemu.

Fix crash when calling screendump from monitor.

This was a regression introduced with r6839:
DisplayAllocator interface (Stefano Stabellini)

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6900 c046a42c-6fe2-441c-8c8c-71466251a162
