qemu: do not copy out non-existent block job info

qemu: do not copy out non-existent block job info

Valgrind complained:

==23975== Conditional jump or move depends on uninitialised value(s)
==23975==    at 0x22255FA6: qemuDomainGetBlockJobInfo (qemu_driver.c:16538)
==23975==    by 0x538E97C: virDomainGetBlockJobInfo (libvirt-domain.c:9685)
==23975==    by 0x12F740: remoteDispatchDomainGetBlockJobInfoHelper (remote.c:2834)
==23975==    by 0x53FF287: virNetServerProgramDispatch (virnetserverprogram.c:437)
==23975==    by 0x540523D: virNetServerProcessMsg (virnetserver.c:135)
==23975==    by 0x54052C7: virNetServerHandleJob (virnetserver.c:156)
==23975==    by 0x52F515B: virThreadPoolWorker (virthreadpool.c:145)
==23975==    by 0x52F4668: virThreadHelper (virthread.c:206)
==23975==    by 0x6E08A50: start_thread (in /lib64/libpthread-2.12.so)
==23975==    by 0x82BE93C: clone (in /lib64/libc-2.12.so)
==23975==
==23975== Conditional jump or move depends on uninitialised value(s)
==23975==    at 0x22255FB4: qemuDomainGetBlockJobInfo (qemu_driver.c:16542)
==23975==    by 0x538E97C: virDomainGetBlockJobInfo (libvirt-domain.c:9685)
==23975==    by 0x12F740: remoteDispatchDomainGetBlockJobInfoHelper (remote.c:2834)
==23975==    by 0x53FF287: virNetServerProgramDispatch (virnetserverprogram.c:437)
==23975==    by 0x540523D: virNetServerProcessMsg (virnetserver.c:135)
==23975==    by 0x54052C7: virNetServerHandleJob (virnetserver.c:156)
==23975==    by 0x52F515B: virThreadPoolWorker (virthreadpool.c:145)
==23975==    by 0x52F4668: virThreadHelper (virthread.c:206)
==23975==    by 0x6E08A50: start_thread (in /lib64/libpthread-2.12.so)
==23975==    by 0x82BE93C: clone (in /lib64/libc-2.12.so)

If no matching block job is found, qemuMonitorGetBlockJobInfo returns 0
and we should not write anything to the caller-supplied
virDomainBlockJobInfo pointer.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>