patch 9.0.0999: memory may leak

Problem:    Memory may leak.
Solution:   Free the sound callback function name if it was allocated.
diff --git a/src/sound.c b/src/sound.c
index 17ff78b..edb606a 100644
--- a/src/sound.c
+++ b/src/sound.c
@@ -60,6 +60,8 @@
 	soundcb->snd_next = first_callback;
 	first_callback = soundcb;
 	set_callback(&soundcb->snd_callback, &callback);
+	if (callback.cb_free_name)
+	    vim_free(callback.cb_name);
     }
     return soundcb;
 }