mutt: Fix arithmetic exception due to menu->pagelen being negative.

[Brendan Cully]

changeset: 6738:69bbe4da959f
user:      Anton Lindqvist <anton.lindqv...@gmail.com>
date:      Tue Jul 19 18:56:13 2016 -0700
link:      http://dev.mutt.org/hg/mutt/rev/69bbe4da959f

Fix arithmetic exception due to menu->pagelen being negative.

Resizing the terminal window down to two lines when in an empty
mailbox causes mutt to crash due to division by zero since menu->max
equals 0 and menu->pagelen < 0 in status.c:205.

Fixing the problem at this specific line felt wrong since I did notice
menu->pagelen being negative. The pagelen is inherited from the rows
calculation in mutt_reflow_windows. Since the number of lines can
potentially be smaller than the accumulated number of rows acquired by
the status, help and message window, make sure the calculation does
not turn negative.

diffs (14 lines):

diff -r f1d5a884ffed -r 69bbe4da959f curs_lib.c
--- a/curs_lib.c        Sun Jul 17 19:31:16 2016 -0700
+++ b/curs_lib.c        Tue Jul 19 18:56:13 2016 -0700
@@ -531,8 +531,8 @@
   MuttMessageWindow->row_offset = LINES - 1;
 
   memcpy (MuttIndexWindow, MuttStatusWindow, sizeof (mutt_window_t));
-  MuttIndexWindow->rows = LINES - MuttStatusWindow->rows - 
MuttHelpWindow->rows -
-                          MuttMessageWindow->rows;
+  MuttIndexWindow->rows = MAX(LINES - MuttStatusWindow->rows -
+                             MuttHelpWindow->rows - MuttMessageWindow->rows, 
0);
   MuttIndexWindow->row_offset = option (OPTSTATUSONTOP) ? 
MuttStatusWindow->rows :
                                                           MuttHelpWindow->rows;