audio: replaced copyAudioFormat() with simple assignment

The "!src" check in copyAudioFormat() used to hide bugs - one should
never pass NULL to it.  There is one caller which might pass NULL, add
a check in this caller.

Instead of doing mempcy(), we can simply assign the structures, which
looks more natural.

src/output_init.c

@@ -103,7 +103,7 @@ int audio_output_init(struct audio_output *ao, ConfigParam * param)
 			FATAL("error parsing format at line %i\n", bp->line);
 		}
 
-		copyAudioFormat(&ao->outAudioFormat, &ao->reqAudioFormat);
+		ao->outAudioFormat = ao->reqAudioFormat;
 	}
 
 	if (plugin->init(ao, param) != 0)