AudioStream: fix invalid memory access

Fix AudioStream::getDeviceName() that calls pa_stream_get_device_name()
without checking the return value.
This last can be the error -PA_ERR_NOTSUPPORTED or null for other issues.
In such cases we return an empty string.

Change-Id: Ia293ff8d93212d87c8fa5e6ce8399964044d530f