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Last-ditch maximum size limit for Flip

This makes sure that width * height <= INT_MAX, which it rather needs
to be.  Also in Flip's case that the square of the area still fits in
an int.

--- a/flip.c

+++ b/flip.c

@@ -8,6 +8,7 @@

 #include <string.h>

 #include <assert.h>

 #include <ctype.h>

+#include <limits.h>

 #include <math.h>

 #include "puzzles.h"

@@ -181,9 +182,16 @@

 static const char *validate_params(const game_params *params, bool full)

 {

+    int wh;

+

     if (params->w <= 0 || params->h <= 0)

         return "Width and height must both be greater than zero";

-    return NULL;

+    if (params->w > (INT_MAX - 3) / params->h)

+        return "Width times height must not be unreasonably large";

+    wh = params->w * params->h;

+    if (wh > (INT_MAX - 3) / wh)

+        return "Width times height is too large";    

+   return NULL;

 }

 static char *encode_bitmap(unsigned char *bmp, int len)