src/output.c: ensure we don't carry around a stale output pointer

src/output.c: ensure we don't carry around a stale output pointer

Prevent carrying around the wlr_output->data pointer when
destroying an output. The set_gamma handler may be called
during the destruction of a wlr_output after our own
destroy handler has already been called. This patch resets
the wlr_output->data pointer in our destroy handler and
adds a check in the set_gamma handler to verify the output
is actually valid.

Fixes #1270

Reported-by: @Flrian