sixel: also set 'alloc_height', when short-cutting a resize operation

In some cases, a sixel may be resized vertically, while still having a
zero-width. In this case, the resize operations are short-cutted, and
no actual allocations are done.

However, we forgot to set 'alloc_height' when doing so. As a result,
the trimming code (when the sixel is "done"), trimmed away the entire
sixel.

CHANGELOG.md

@@ -64,6 +64,9 @@
 
 * Crash when printing double-width (or longer) characters to, or near,
   the last column, when auto-wrap (private mode 7) has been disabled.
+* Dynamically sized sixel being trimmed to nothing.
+
+
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