PcmMix: use negative value instead of NaN for addition

Avoid NaN to allow -ffast-math.

src/pcm/PcmMix.cxx

@@ -194,9 +194,9 @@ pcm_mix(void *buffer1, const void *buffer2, size_t size,
 	int vol1;
 	float s;
 
-	/* portion1 is between 0.0 and 1.0 for crossfading, MixRamp uses NaN
+	/* portion1 is between 0.0 and 1.0 for crossfading, MixRamp uses -1
 	 * to signal mixing rather than fading */
-	if (isnan(portion1))
+	if (portion1 < 0)
 		return pcm_add(buffer1, buffer2, size, format);
 
 	s = sin(M_PI_2 * portion1);

src/pcm/PcmMix.hxx

@@ -36,8 +36,9 @@
  * @param size the size of both buffers in bytes
  * @param format the sample format of both buffers
  * @param portion1 a number between 0.0 and 1.0 specifying the portion
- * of the first buffer in the mix; portion2 = (1.0 - portion1). The value
- * NaN is used by the MixRamp code to specify that simple addition is required.
+ * of the first buffer in the mix; portion2 = (1.0 - portion1).
+ * Negative values are used by the MixRamp code to specify that simple
+ * addition is required.
  *
  * @return true on success, false if the format is not supported
  */