Double11 fix2 (#2937)

* [iOS] Fix issue JS thread may be deallocated not in JS thread.

* [iOS] Fix the issue that iOS13 logic not working if config center is not set.

ios/sdk/WeexSDK/Sources/Bridge/WXJSCoreBridge.mm

@@ -80,6 +80,7 @@ - (void)dealloc
 {
     _jsContext.instanceId = nil;
     __block JSContext* theContext = _jsContext;
+    _jsContext = nil; // Make sure that the context MUST be freed in JS thread.
     WXPerformBlockOnBridgeThreadForInstance(^{
          theContext = nil; // release the context in js thread to avoid main-thread deadlock
     }, _weexInstanceId);

ios/sdk/WeexSDK/Sources/Model/WXSDKInstance.m

@@ -606,6 +606,10 @@ - (BOOL)_handleConfigCenter
         BOOL alwaysUseMRCForObjectToWeexCore = [[configCenter configForKey:@"iOS_weex_ext_config.alwaysUseMRC" defaultValue:@(NO) isDefault:NULL] boolValue];
         ConvertSwitches(isIOS13, useMRCForInvalidJSONObject, alwaysUseMRCForObjectToWeexCore);
     }
+    else {
+        BOOL isIOS13 = [[[UIDevice currentDevice] systemVersion] integerValue] == 13;
+        ConvertSwitches(isIOS13, YES, NO);
+    }
     return NO;
 }