BUG/MINOR debug: fix !USE_THREAD_DUMP in ha_thread_dump_fill()

BUG/MINOR debug: fix !USE_THREAD_DUMP in ha_thread_dump_fill()

The function must make sure to return NULL for foreign threads and
the local buffer for the current thread in this case, otherwise panics
(and sometimes even warnings) will segfault when USE_THREAD_DUMP is
disabled. Let's slightly re-arrange the function to reduce the #if/else
since we have to specifically handle the case of !USE_THREAD_DUMP anyway.

This needs to be backported wherever b8adef065d ("MEDIUM: debug: on
panic, make the target thread automatically allocate its buf") was
backported (at least 2.8).