Set the terminal tilegrid NULL after free
Set the terminal tilegrid NULL after free

Without this, a double free can occur when a display (and terminal)
is released and then a crash occurs. Upon a second release,
different memory is released (sometimes the heap). When this is
followed by an allocation for the flash cache, the cache can
overwrite the active heap causing crashes.

Fixes #1667

adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython