storage: Fix coverity warning

Introduced by commit e0139e30:

1777 	    /* Updating pool metadata */

(40) Event var_deref_op: Dereferencing null pointer "newvol".
     Also see events: [assign_zero]

1778 	    pool->def->allocation += newvol->allocation;
1779 	    pool->def->available -= newvol->allocation;

src/storage/storage_driver.c

@@ -1635,6 +1635,7 @@ storageVolCreateXMLFrom(virStoragePoolPtr obj,
     virStorageBackendPtr backend;
     virStorageVolDefPtr origvol = NULL, newvol = NULL;
     virStorageVolPtr ret = NULL, volobj = NULL;
+    unsigned long long allocation;
     int buildret;
 
     virCheckFlags(VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA, NULL);
@@ -1758,6 +1759,7 @@ storageVolCreateXMLFrom(virStoragePoolPtr obj,
 
     origvol->building = 0;
     newvol->building = 0;
+    allocation = newvol->allocation;
     newvol = NULL;
     pool->asyncjobs--;
 
@@ -1775,8 +1777,8 @@ storageVolCreateXMLFrom(virStoragePoolPtr obj,
     }
 
     /* Updating pool metadata */
-    pool->def->allocation += newvol->allocation;
-    pool->def->available -= newvol->allocation;
+    pool->def->allocation += allocation;
+    pool->def->available -= allocation;
 
     VIR_INFO("Creating volume '%s' in storage pool '%s'",
              volobj->name, pool->def->name);