Refs #1413: Avoid infinite loops when restoring the iterator position.

src/Framework/Constraint/Count.php

@@ -106,7 +106,7 @@ protected function getCountOf($other)
             // moves pointer
             if ($key !== null) {
                 $iterator->rewind();
-                while ($key !== $iterator->key()) {
+                while ($iterator->valid() && $key !== $iterator->key()) {
                     $iterator->next();
                 }
             }