Improve encoder example to reject audio without any channels.

This avoid a crash on bogus input, just exit with a message instead.

Fixes #2305

examples/encoder_example.c

@@ -33,6 +33,7 @@
 #endif
 /*#define OC_COLLECT_METRICS*/
 
+#include <assert.h>
 #include <stdio.h>
 #if !defined(_WIN32)
 #include <getopt.h>
@@ -775,6 +776,10 @@ static void id_file(char *f){
 
           audio=test;
           audio_ch=buffer[6]+(buffer[7]<<8);
+          if (0 >= audio_ch) {
+            fprintf(stderr,"Can only read WAV files with non-zero audio channels for now.\n");
+            exit(1);
+          }
           audio_hz=buffer[8]+(buffer[9]<<8)+
             (buffer[10]<<16)+(buffer[11]<<24);