Fetch fix for userData (#11729)

The userData field is present in both the emscripten_fetch_t and
emscripten_fetch_attr_t objects - once we have a fetch going on, it
should be read from the fetch instance object instead.

Author: 태재영

src/Fetch.js

@@ -291,7 +291,7 @@ function __emscripten_fetch_xhr(fetch, onsuccess, onerror, onprogress, onreadyst
   var fetch_attr = fetch + {{{ C_STRUCTS.emscripten_fetch_t.__attributes }}};
   var requestMethod = UTF8ToString(fetch_attr);
   if (!requestMethod) requestMethod = 'GET';
-  var userData = HEAPU32[fetch_attr + {{{ C_STRUCTS.emscripten_fetch_attr_t.userData }}} >> 2];
+  var userData = HEAPU32[fetch + {{{ C_STRUCTS.emscripten_fetch_t.userData }}} >> 2];
   var fetchAttributes = HEAPU32[fetch_attr + {{{ C_STRUCTS.emscripten_fetch_attr_t.attributes }}} >> 2];
   var timeoutMsecs = HEAPU32[fetch_attr + {{{ C_STRUCTS.emscripten_fetch_attr_t.timeoutMSecs }}} >> 2];
   var withCredentials = !!HEAPU32[fetch_attr + {{{ C_STRUCTS.emscripten_fetch_attr_t.withCredentials }}} >> 2];