sixel: don’t crash when sixel image exceeds current sixel max height

When we try to resize a sixel past the current max height, we set
col > image-width to signal this.

This means ‘width’ could be smaller than ‘col’. When calculating how
many sixels to emit in sixel_add_many(), we didnt’ account for this.

The resulting value was -1, converted to ‘unsigned’. I.e. a very large
value. This resulted in an assert triggering in sixel_add() in debug
builds, and a crash in release builds.

CHANGELOG.md

@@ -88,6 +88,7 @@
   built-in terminfo (accessed via XTGETTCAP).
 * Crash when interactively resizing the window with a very large
   scrollback.
+* Crash when a sixel image exceeds the current sixel max height.
 
 [1173]: https://codeberg.org/dnkl/foot/issues/1173
 

sixel.c

@@ -1295,7 +1295,7 @@ sixel_add_many(struct terminal *term, uint8_t c, unsigned count)
     if (unlikely(col + count - 1 >= width)) {
         resize_horizontally(term, col + count);
         width = term->sixel.image.width;
-        count = min(count, width - col);
+        count = min(count, max(width - col, 0));
     }
 
     uint32_t color = term->sixel.color;