Revert "qapi: Clean up superfluous null check in qapi_dealloc_type_str()"

Revert "qapi: Clean up superfluous null check in qapi_dealloc_type_str()"

This reverts commit 25a7017555f1b4aeb543b5d323ff4afb8f9c5437.

Turns out the argument *can* be null: QEMU now segfaults if it
receives an invalid parameter via a qmp command instead of throwing an
error.

For example:
{ "execute": "blockdev-add",
     "arguments": { "options" : { "driver": "invalid-driver" } } }

CC: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
(cherry picked from commit b690d679c1ca65d71b0544a2331d50e9f0f95116)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>