avoid FPE when number of channels is zero. Fix MestreLion#6

Author: MestreLion

audio.c

@@ -800,7 +800,9 @@ int wav_open(FILE *in, wavegain_opt *opt,
 							of trying to abstract stuff. */
 		wav->samplesize = format.samplesize;
 
-		if (len)
+		if (!format.channels)
+			opt->total_samples_per_channel = 0;
+		else if (len)
 			opt->total_samples_per_channel = len/(format.channels*samplesize);
 		else {
 			Int64_t pos;

wavegain.c

@@ -192,7 +192,8 @@ int get_gain(const char *filename, double *track_peak, double *track_gain,
 	}
 
 	if ((wg_opts->channels != 1) && (wg_opts->channels != 2)) {
-		fprintf(stderr, " Unsupported number of channels.\n");
+		fprintf(stderr, " Unsupported number of channels (%d) for %s.\n",
+				wg_opts->channels, filename);
 		goto exit;
 	}