qemu: Check if virQEMUCapsNewCopy(...) has failed

Check if virQEMUCapsNewCopy(...) has failed, thus a segmentation fault
in virQEMUCapsFilterByMachineType(...) will be avoided.
Signed-off-by: NMarc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.vnet.ibm.com>

src/qemu/qemu_capabilities.c

@@ -4928,6 +4928,10 @@ virQEMUCapsCacheLookupCopy(virCapsPtr caps,
 
     ret = virQEMUCapsNewCopy(qemuCaps);
     virObjectUnref(qemuCaps);
+
+    if (!ret)
+        return NULL;
+
     virQEMUCapsFilterByMachineType(ret, machineType);
     return ret;
 }