Don't try to mix a sample that has already ended

On short samples the sample may finish playing before the mixer is done.
This fills the remaining time with zeros and ends mixing. This fixes the
users getting the following error logged:

::_mix_internal: Condition ' !active ' is true.

servers/audio/audio_stream.cpp

@@ -76,6 +76,13 @@ void AudioStreamPlaybackResampled::mix(AudioFrame *p_buffer, float p_rate_scale,
 			internal_buffer[1] = internal_buffer[INTERNAL_BUFFER_LEN + 1];
 			internal_buffer[2] = internal_buffer[INTERNAL_BUFFER_LEN + 2];
 			internal_buffer[3] = internal_buffer[INTERNAL_BUFFER_LEN + 3];
+			if (!is_playing()) {
+				for (int i = 4; i < INTERNAL_BUFFER_LEN; ++i) {
+					internal_buffer[i] = AudioFrame(0, 0);
+				}
+
+				return;
+			}
 			_mix_internal(internal_buffer + 4, INTERNAL_BUFFER_LEN);
 			mix_offset -= (INTERNAL_BUFFER_LEN << FP_BITS);
 		}