[PATCH v2] Fix segfault when restoring w_alt.cursor

Screen can be segfaulted with the following steps:

  - Start screen
  - Enable altscreen with ":altscreen on"
  - 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

This happens because if the canvas height is 0, the alt cursor will be
adjusted to he - 1 == -1. When we later restore this cursor screen
crashes. I've reordered the sanity checks for w_saved in case there is
a similar bug for the saved cursor.
---

ChangeLog

v2 - This is a less paranoid version that is arguably more correct, since
it allows screen to render correctly when the only available line is
taken by the caption.

 src/resize.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/resize.c b/src/resize.c
index 0e491eb..3c2b90f 100644
--- a/src/resize.c
+++ b/src/resize.c
@@ -966,14 +966,16 @@ int wi, he, hi;
     p->w_y = he - 1;
   if (p->w_saved.x > wi)
     p->w_saved.x = wi;
-  if (p->w_saved.y < 0)
-    p->w_saved.y = 0;
   if (p->w_saved.y >= he)
     p->w_saved.y = he - 1;
+  if (p->w_saved.y < 0)
+    p->w_saved.y = 0;
   if (p->w_alt.cursor.x > wi)
     p->w_alt.cursor.x = wi;
   if (p->w_alt.cursor.y >= he)
     p->w_alt.cursor.y = he - 1;
+  if (p->w_alt.cursor.y < 0)
+    p->w_alt.cursor.y = 0;
 
   /* reset scrolling region */
   p->w_top = 0;
-- 
1.7.7.3