qemu: Check virGetLastError return value for migration finish failure

Commit id '2e7cea24' added a check for an error from Finish instead
of 'unexpected error'; however, if for some reason there wasn't an
error, then virGetLastError could return NULL resulting in the
NULL pointer deref to err->domain.

src/libvirt-domain.c

@@ -3195,7 +3195,8 @@ virDomainMigrateVersion3Full(virDomainPtr domain,
                 orig_err->domain == VIR_FROM_QEMU &&
                 orig_err->code == VIR_ERR_OPERATION_FAILED) {
                 virErrorPtr err = virGetLastError();
-                if (err->domain == VIR_FROM_QEMU &&
+                if (err &&
+                    err->domain == VIR_FROM_QEMU &&
                     err->code != VIR_ERR_MIGRATE_FINISH_OK) {
                     virFreeError(orig_err);
                     orig_err = NULL;

src/qemu/qemu_migration.c

@@ -5023,7 +5023,8 @@ doPeer2PeerMigrate3(virQEMUDriverPtr driver,
                 orig_err->domain == VIR_FROM_QEMU &&
                 orig_err->code == VIR_ERR_OPERATION_FAILED) {
                 virErrorPtr err = virGetLastError();
-                if (err->domain == VIR_FROM_QEMU &&
+                if (err &&
+                    err->domain == VIR_FROM_QEMU &&
                     err->code != VIR_ERR_MIGRATE_FINISH_OK) {
                     virFreeError(orig_err);
                     orig_err = NULL;