fix bug.

@@ -25,7 +25,7 @@
 int
 mbox_set_progress_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   mbox->progress.cb = cb;
   mbox->progress.arg = arg;
@@ -35,7 +35,7 @@ mbox_set_progress_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 int
 mbox_set_newmsg_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   mbox->newmsg.cb = cb;
   mbox->newmsg.arg = arg;
@@ -45,7 +45,7 @@ mbox_set_newmsg_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 int
 mbox_set_error_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   mbox->error.cb = cb;
   mbox->error.arg = arg;
@@ -55,7 +55,7 @@ mbox_set_error_cb (mbox_t mbox, int (*cb) __P ((int, void *)), void *arg)
 int
 mbox_newmsg_cb (mbox_t mbox, int info)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   if (mbox->newmsg.cb)
     return mbox->newmsg.cb (info, mbox->newmsg.arg);
@@ -65,7 +65,7 @@ mbox_newmsg_cb (mbox_t mbox, int info)
 int
 mbox_progress_cb (mbox_t mbox, int info)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   if (mbox->progress.cb)
     return mbox->progress.cb (info, mbox->progress.arg);
@@ -75,7 +75,7 @@ mbox_progress_cb (mbox_t mbox, int info)
 int
 mbox_error_cb (mbox_t mbox, int info)
 {
-  if (mbox)
+  if (mbox == NULL)
     return MU_ERROR_INVALID_PARAMETER;
   if (mbox->error.cb)
     return mbox->error.cb (info, mbox->error.arg);

@@ -176,6 +176,8 @@ mbox_scan0 (mbox_t mbox, unsigned int msgno, unsigned int *pcount,
       else
 	status = MU_ERROR_IO;
     }
+  else
+    status = MU_ERROR_IO;
 
   /* Move along, move along nothing to see.  */
   if (!mbox_has_newmail (mbox) && mbox->size == file_size)