Fix linter errors in mock_engine (#21102)
Fix linter errors in mock_engine (#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.
rmacnak-google/engine,jamesr/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,Hixie/sky_engine,devoncarew/sky_engine,devoncarew/sky_engine,aam/engine,Hixie/sky_engine,jamesr/flutter_engine,jason-simmons/sky_engine,aam/engine,chinmaygarde/flutter_engine,Hixie/sky_engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,flutter/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,aam/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,aam/engine,devoncarew/sky_engine,chinmaygarde/sky_engine,jamesr/sky_engine,rmacnak-google/engine,devoncarew/engine,aam/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,flutter/engine,flutter/engine,jamesr/flutter_engine,Hixie/sky_engine,chinmaygarde/sky_engine,jamesr/sky_engine,jamesr/sky_engine,devoncarew/sky_engine,aam/engine,jason-simmons/flutter_engine,rmacnak-google/engine,Hixie/sky_engine,jamesr/sky_engine,jason-simmons/sky_engine,aam/engine,devoncarew/engine,flutter/engine,flutter/engine,aam/engine,devoncarew/engine,devoncarew/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,devoncarew/engine,rmacnak-google/engine,jason-simmons/sky_engine,devoncarew/engine,flutter/engine,chinmaygarde/flutter_engine,jamesr/sky_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,rmacnak-google/engine,jason-simmons/flutter_engine,jason-simmons/sky_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,flutter/engine,flutter/engine,jason-simmons/sky_engine,jamesr/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,rmacnak-google/engine,jamesr/sky_engine,devoncarew/sky_engine,jason-simmons/sky_engine,devoncarew/engine,chinmaygarde/sky_engine