ruby : fix dangling pointers, memory leak, and SEGV on parallel transcription (#3715)

ruby : fix dangling pointers, memory leak, and SEGV on parallel transcription (#3715)

* Prevent dangling pointers

* Use proper free function

* Free callback containers

* Set default log callback when nil is passed to log_set

* Raise error if callbacks set when parallel transcription

* Bump version to 1.3.7

* Make tests follow spec change

* Add note on parallel transcription and callbacks

* Update signature of Whisper.log_set [skip ci]