Fix warning messages.

garglk/sndsdl.c

@@ -431,7 +431,7 @@ static void sound_completion_callback(int chan)
 
     if (!sound_channel)
     {
-        fprintf(stderr, "sound_completion_callback called with invalid sound_channel %d\n", chan);
+        gli_strict_warning("sound completion callback called with invalid channel");
         return;
     }
 
@@ -650,7 +650,10 @@ static glui32 play_mod(schanid_t chan, long len)
     int music_busy, loop;
 
     if (chan == NULL)
+    {
         gli_strict_warning("MOD player called with an invalid channel!");
+        return 0;
+    }
 
     music_busy = Mix_PlayingMusic();
 
@@ -754,8 +757,6 @@ glui32 glk_schannel_play_ext(schanid_t chan, glui32 snd, glui32 repeats, glui32
     /* if channel was paused it should be paused again */
     if (result && paused)
     {
-        if (paused)
-            fprintf(stderr, "glk_schannel_play_ext: pausing channel again\n");
         glk_schannel_pause(chan);
     }