[PATCH] Forbid creation of zero-height canvas

The code is not prepared to handle a canvas with no lines. One way to
segfault screen with a 0-line canvas is:

  - Start screen
  - Enable caption with ":caption always"
  - Run vim
  - Resize the current terminal to one line high
  - Resize the current terminal to multiple lines high
  - Quit vim

If the window is too small then don't expect this to render ok,
but it does prevent screen from crashing.
---
 src/resize.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/src/resize.c b/src/resize.c
index 0e491eb..0a911db 100644
--- a/src/resize.c
+++ b/src/resize.c
@@ -153,13 +153,20 @@ int change_fore;
   struct win *p;
   struct canvas *cv;
   int wwi;
+  int minhe;
 
   debug2("ChangeScreenSize from (%d,%d) ", D_width, D_height);
   debug3("to (%d,%d) (change_fore: %d)\n",wi, he, change_fore);
 
   cv = &D_canvas;
+  minhe = 1 + ((cv->c_slperp && cv->c_slperp->c_slnext) || captionalways) + (D_has_hstatus == HSTATUS_LASTLINE);
+  if (he < minhe)
+    {
+      debug1("too small, new height: %d\n", minhe);
+      he = minhe;
+    }
   cv->c_xe = wi - 1;
-  cv->c_ye = he - 1 - ((cv->c_slperp && cv->c_slperp->c_slnext) || captionalways) - (D_has_hstatus == HSTATUS_LASTLINE);
+  cv->c_ye = he - minhe;
   cv->c_blank.l_height = cv->c_ye - cv->c_ys + 1;
   if (cv->c_slperp)
     {
-- 
1.7.7.3