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: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_driver.c

@@ -20728,7 +20728,8 @@ qemuDomainGetResctrlMonData(virQEMUDriverPtr driver,
     size_t i = 0;
     size_t j = 0;
 
-    caps = virQEMUDriverGetCapabilities(driver, false);
+    if (!(caps = virQEMUDriverGetCapabilities(driver, false)))
+        return -1;
 
     switch (tag) {
     case VIR_RESCTRL_MONITOR_TYPE_CACHE: