#25 segault caused by lack of error checking

[bugant]

In audio_portaudio.c pa_start return
values are never checked.

This could cause a segfault if the audio divece
is already in use.

The bug is reproducible on ubuntu 6.04
"Dapper Drake":

1) Start to play a song
2) Start using iaxclient:

* iaxc_initialize(AUDIO_INTERNAL_PA,1)
* iaxc_set_silence_threshold(-99.0);
* iaxc_set_audio_output(0);
* iaxc_set_event_callback(callback);
* iaxc_start_processing_thread()

segfault

PortAudio error at Unable to open streams: Illegal
error number.
Segmentation Fault

[bugant]

patch to fix the bug: checking for pa_start return value