Initialize callback for safety


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@8411 a1c6a512-1295-4272-9138-f99709370657
diff --git a/apps/pcmbuf.c b/apps/pcmbuf.c
index d1835fb..fc58d9c 100644
--- a/apps/pcmbuf.c
+++ b/apps/pcmbuf.c
@@ -275,6 +275,7 @@
     audiobuffer = (char *)&audiobuf[(audiobufend - audiobuf) - 
                                     pcmbuf_size - PCMBUF_GUARD];
     guardbuf = &audiobuffer[pcmbuf_size];
+    position_callback = NULL;
     pcmbuf_event_handler = NULL;
     pcmbuf_play_stop();
 }
@@ -327,6 +328,7 @@
                 return ;
             }
         }
+        position_callback = NULL;
         pcmbuf_event_handler = NULL;
         audiobuffer_pos += copy_n;
         if (audiobuffer_pos >= pcmbuf_size)