It was incorrect to NULL out the pointer to our heap allocated buffer in
`reset`, because subsequent to framebuffer_reset, but while
the heap was still active, we could call `get_bufinfo` again,
leading to a fresh allocation on the heap that is about to be destroyed.
Typical stack trace:
```
#1 0x0006c368 in sharpdisplay_framebuffer_get_bufinfo
#2 0x0006ad6e in _refresh_display
#3 0x0006b168 in framebufferio_framebufferdisplay_background
#4 0x00069d22 in displayio_background
#5 0x00045496 in supervisor_background_tasks
#6 0x000446e8 in background_callback_run_all
#7 0x00045546 in supervisor_run_background_tasks_if_tick
#8 0x0005b042 in common_hal_neopixel_write
#9 0x00044c4c in clear_temp_status
#10 0x000497de in spi_flash_flush_keep_cache
#11 0x00049a66 in supervisor_external_flash_flush
#12 0x00044b22 in supervisor_flash_flush
#13 0x0004490e in filesystem_flush
#14 0x00043e18 in cleanup_after_vm
#15 0x0004414c in run_repl
#16 0x000441ce in main
```
When this happened -- which was inconsistent -- the display would keep
some heap allocation across reset which is exactly what we need to avoid.
NULLing the pointer in reconstruct follows what RGBMatrix does, and that
code is a bit more battle-tested anyway.
If I had a motivation for structuring the SharpMemory code differently,
I can no longer recall it.
Testing performed: Ran my complicated calculator program over multiple
iterations without observing signs of heap corruption.
Closes: #3473
Jeff Epler
d7c3f811af