qemu: avoid null deref on block pull error

qemu: avoid null deref on block pull error

Coverity detected that 5 of 6 callers of virJSONValueArrayGet checked
for a NULL return; and that by not checking we risk a null deref
during an error.  The error is unlikely since the prior call to
virJSONValueArraySize would probably have already caught any botched
JSON array parse, but better safe than sorry.

* src/qemu/qemu_monitor_json.c (qemuMonitorJSONGetBlockJobInfo):
Check for NULL.
(qemuMonitorJSONExtractPtyPaths): Fix typo.