don't change state in case of exception

Author: xabbuh

ValidatorBuilder.php

@@ -255,12 +255,12 @@ public function setMetadataCache(CacheInterface $cache)
      */
     public function setMappingCache(CacheItemPoolInterface $cache)
     {
-        $this->mappingCache = $cache;
-
         if (null !== $this->metadataFactory) {
             throw new ValidatorException('You cannot set a custom mapping cache after setting a custom metadata factory. Configure your metadata factory instead.');
         }
 
+        $this->mappingCache = $cache;
+
         return $this;
     }