qapi: Fix potential NULL pointer segfault

qapi: Fix potential NULL pointer segfault

Report from smatch:

qapi-visit.c:1640 visit_type_BlockdevAction(8) error:
 we previously assumed 'obj' could be null (see line 1639)
qapi-visit.c:2432 visit_type_NetClientOptions(8) error:
 we previously assumed 'obj' could be null (see line 2431)

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>