fix(DefinitionResolver): don't crash if foreach key isn't a variable (#564)

Author: phil-nelson

fixtures/completion/foreach.php

@@ -36,5 +36,5 @@ public function test() { }
     $
 }
 
-foreach ($unknownArray as $unknown) {
+foreach ($unknownArray as $member->access => $unknown) {
     $unkno

src/DefinitionResolver.php

@@ -571,7 +571,9 @@ public function resolveVariableToNode($var)
 
             // If we get to a ForeachStatement, check the keys and values
             if ($n instanceof Node\Statement\ForeachStatement) {
-                if ($n->foreachKey && $n->foreachKey->expression->getName() === $name) {
+                if ($n->foreachKey instanceof Node\Expression\Variable
+                    && $n->foreachKey->expression->getName() === $name
+                ) {
                     return $n->foreachKey;
                 }
                 if ($n->foreachValue