sixel: prevent allocating an absurd amount of memory or writing OOB (#20213)

This commit implements two fixes for the integer overflow/out-of-bounds
write reported in #20149.

First, it catches any exception generated in sixel char processing
(which will prevent `out_of_memory` or `bad_alloc` from being ignored
sight-unseen, and will prevent the consumption of further DCS content).

Second, it prevents us from allocating memory for an image which will
never be displayed (because it exceeds the height of the display.)

This supersedes prior work in #20153 for the same issues.

Closes #20149
Closes #20153

Co-authored-by: James Holderness <j4_james@hotmail.com>