Fix md_get_all() return value (#1194)

It is a follow-up PR for #1187 that corrects retval of internal
`md_get_all()` function.

Author: asvetlov

multidict/_multilib/hashtable.h

@@ -868,7 +868,7 @@ md_get_all(MultiDictObject *md, PyObject *key, PyObject **ret)
 
     md_finder_cleanup(&finder);
     Py_DECREF(identity);
-    return ret != NULL;
+    return *ret != NULL;
 fail:
     md_finder_cleanup(&finder);
     Py_XDECREF(identity);