warning - fix undefined array key labels (#4777)

Author: gdemarsico

app/code/core/Mage/ConfigurableSwatches/Helper/Productimg.php

@@ -376,7 +376,7 @@ public function filterImageInGallery($product, $image)
 
         if (!isset($this->_productImageFilters[$product->getId()])) {
             $mapping = call_user_func_array('array_merge_recursive', array_values($product->getChildAttributeLabelMapping()));
-            $filters = array_unique($mapping['labels']);
+            $filters = isset($mapping['labels']) ? array_unique($mapping['labels']) : [];
             $filters = array_merge($filters, array_map(function ($label) {
                 return $label . Mage_ConfigurableSwatches_Helper_Productimg::SWATCH_LABEL_SUFFIX;
             }, $filters));