tracing: fix mmiotrace resizing crash

Pekka reported a crash when resizing the mmiotrace tracer (if only
mmiotrace is enabled).

This happens because in that case we do not allocate the max buffer,
but we try to use it.

Make ring_buffer_resize() idempotent against NULL buffers.

Reported-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>