pulse-server: avoid skipping more ringbuffer data than available

This prevents the ringbuffer from advancing the read pointer more than
the size of data actually available. It prevents the "avail" value from
keeping to drop when no audio is being played by the client.

Applying this patch seems to prevent the "randomly playing music after
a couple hours of silence" issue

Fixes #2366

src/modules/module-protocol-pulse/pulse-server.c

@@ -1288,10 +1288,10 @@ static void stream_process(void *data)
 						stream->buffer, MAXLENGTH,
 						index % MAXLENGTH,
 						p, avail);
+					index += avail;
 				}
 				pd.playing_for = size;
 				pd.read_inc = size;
-				index += size;
 				spa_ringbuffer_read_update(&stream->ring, index);
 			}
 			pw_log_debug("%p: [%s] underrun read:%u avail:%d max:%u",