#3507 papers over a problem where we called self->bus_free(self->bus); when bus_free was a NULL function pointer. In the case of RGBMatrix, this is fine. In the case of SharpDisplay, however, there is the possibility that the underlying SPI bus will be locked and we need to deal with it.

This could be done by

• adding a new Framebuffer protocol to check for bus free (default/NULL function indicates bus is always free)
  • and implementing it for SharpDisplay, checking that the bus is in fact free
• assigning this protocol to display_core.bus_free
• removing the NULL check I added for self->bus

Presumably, without this fix, it is possible for a lockup to happen with an auto-refresh SharpDisplay and another foreground use of the SPI bus, including SD cards. However, I did not actually test anything in this area.