Fix DOMException in _.merge. [closes #895]

lodash.src.js

@@ -2537,6 +2537,9 @@
             ? toPlainObject(value)
             : (isPlainObject(value) ? value : {});
         }
+        else {
+          isCommon = false;
+        }
       }
       // Add the source value to the stack of traversed objects and associate
       // it with its merged value.