qemu: Fix possible NULL deref in qemuDomainGetResctrlMonData
qemu: Fix possible NULL deref in qemuDomainGetResctrlMonData

If virQEMUDriverGetCapabilities returns NULL, then a subsequent
deref of @caps would cause an error, so we just return failure.

Found by Coverity

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

nertpinx/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,andreabolognani/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,jardasgit/libvirt,andreabolognani/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jardasgit/libvirt