Add a missing NULL check.

The get_next_write_timestamp method introduced to the audio HAL is optional.
HALs which do not implement it leave it set to NULL.  Callers (there is
currently only one in the AudioMixer code) need to be certain to check for NULL
before invoking it.

Change-Id: I88ba43bb53bec081c98c9a8842936c4fbfdd44f6