Make opusfile_example fwrites all-or-nothing.

This makes it more likely the WAV file header matches the amount of
 data actually written if we run out of space.
This is easier than making the accounting better.

examples/opusfile_example.c

@@ -301,7 +301,7 @@ int main(int _argc,const char **_argv){
         out[2*si+0]=(unsigned char)(pcm[si]&0xFF);
         out[2*si+1]=(unsigned char)(pcm[si]>>8&0xFF);
       }
-      if(!fwrite(out,sizeof(*out)*4,ret,stdout)){
+      if(!fwrite(out,sizeof(*out)*4*ret,1,stdout)){
         fprintf(stderr,"\nError writing decoded audio data: %s\n",
          strerror(errno));
         ret=EXIT_FAILURE;