audio: Do not segfault in out_get_presentation_position()

When a voice call is stopped we switch back to the default primary
output (speaker). Then this function gets executed and as ther was
no active PCM because voice_session is handling that it segfaults
because the PCM is NULL.

Change-Id: I927504b7962b096c0d4c3642b48aee55c85ec013
1 file changed