USBMIDI::write() compiles but doesn't work on Raspberry Pi Pico

Raspberry Pi Pico board support includes the USBMIDI.h header from MBed OS, but it doesn't seem to work. When USBMIDI::write is called it prints a garbage character to the serial console and crashes.

I expected it either to work or result in some kind of compile error.

Sample code attached.

[pressure_sensor_test.ino.txt](https://github.com/arduino/ArduinoCore-mbed/files/6527612/pressure_sensor_test.ino.txt)

Start with the attached pressure_sensor_test.ino.txt sample and the Raspberry Pi Pico USBMIDI.h implementation, then trace USBMIDI::write and its serial output. Reproduce the garbage character and crash on the Pico; done means the call works correctly or fails with a clear compile-time error instead.

Written by the indexing model from the issue text.