storage: Clean up error path for create buildPool failure

Commit id 'aeb1078ab' added a buildPool option and failure path which
calls virStoragePoolObjRemove, which unlocks the pool, clears the 'pool'
variable, and goto cleanup.  However, at cleanup virStoragePoolObjUnlock
is called without check if pool is non NULL.

src/storage/storage_driver.c

@@ -930,7 +930,8 @@ storagePoolCreate(virStoragePoolPtr obj,
 
  cleanup:
     VIR_FREE(stateFile);
-    virStoragePoolObjUnlock(pool);
+    if (pool)
+        virStoragePoolObjUnlock(pool);
     return ret;
 }