Fix UTF8 validation crash on Xamarin
Log messages were passed, unchecked, to the log_message_written callback
On Xamarin this callback is written in C#. So the const char* is converted to a .NET string, which is stored as UTF16. Mono checks for validity and throws an exception when the string contains illegal byte sequences, leading to a crash.
Interestingly, though, I was unable to reproduce the crash on UWP, which uses the .NET Core runtime. The illegal bytes seem to be silently replaced with the replacement character ('�') without throwing.
Parent: linphone-sdk!2877 (merged)
Backport of: !2006 (merged)