Fix linter errors in mock_engine (flutter#21102)

Make a single-param ctor explicit in order to prevent surprising
implicit conversions.

Add a check for zero message-size and don't malloc/memcpy the incoming
message in those cases.

Add braces where they were missing.

Author: cbracken

shell/platform/linux/testing/mock_engine.cc

@@ -32,7 +32,7 @@ struct _FlutterPlatformMessageResponseHandle {
   bool released;
 
   // Constructor for a response handle generated by the engine.
-  _FlutterPlatformMessageResponseHandle(std::string channel)
+  explicit _FlutterPlatformMessageResponseHandle(std::string channel)
       : data_callback(nullptr),
         user_data(nullptr),
         channel(channel),
@@ -61,8 +61,12 @@ struct _FlutterTaskRunner {
         channel(channel),
         response_handle(response_handle),
         message_size(message_size) {
-    this->message = static_cast<uint8_t*>(malloc(message_size));
-    memcpy(this->message, message, message_size);
+    if (message_size > 0) {
+      this->message = static_cast<uint8_t*>(malloc(message_size));
+      memcpy(this->message, message, message_size);
+    } else {
+      this->message = nullptr;
+    }
   }
   ~_FlutterTaskRunner() {
     if (response_handle != nullptr) {
@@ -80,8 +84,9 @@ static void send_response(
     const FlutterPlatformMessageResponseHandle* response_handle,
     const uint8_t* message,
     size_t message_size) {
-  if (response_handle == nullptr)
+  if (response_handle == nullptr) {
     return;
+  }
 
   FlutterTask task;
   task.runner = new _FlutterTaskRunner(1234, channel, response_handle, message,
@@ -146,8 +151,9 @@ FlutterEngineResult FlutterEngineRun(size_t version,
 
   FlutterEngineResult result =
       FlutterEngineInitialize(version, config, args, user_data, engine_out);
-  if (result != kSuccess)
+  if (result != kSuccess) {
     return result;
+  }
   return FlutterEngineRunInitialized(*engine_out);
 }
 
@@ -254,8 +260,9 @@ FlutterEngineResult FlutterEngineSendPlatformMessage(
                       ? fl_value_get_string(message_value)
                       : nullptr;
       }
-      if (fl_value_get_length(args) >= 3)
+      if (fl_value_get_length(args) >= 3) {
         details = fl_value_get_list_value(args, 2);
+      }
       response = fl_method_codec_encode_error_envelope(
           FL_METHOD_CODEC(codec), code, message, details, &error);
       EXPECT_EQ(error, nullptr);
@@ -336,8 +343,9 @@ FlutterEngineResult FlutterEngineSendPlatformMessageResponse(
   EXPECT_TRUE(engine->running);
 
   // Send a message so the shell can check the responses received.
-  if (handle->channel != "test/responses")
+  if (handle->channel != "test/responses") {
     send_message(engine, "test/responses", data, data_length);
+  }
 
   EXPECT_FALSE(handle->released);