Fix FS#12241. Voice stopped working after audio_get_buffer() calls.
Since r30308 the talk buffer was set to NULL if e.g. a plugin called
audio_get_buffer() to steal the talk buffer. Since there's no audio_release_buffer() kind of function
the talk buffer was never set back again.
When trying to talk try to get the audio buffer with audio_get_buffer() as well,
which works until the audio buffer gets properly reinitialized.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30840 a1c6a512-1295-4272-9138-f99709370657