patch 9.2.0076: [security]: buffer-overflow in terminal handling

patch 9.2.0076: [security]: buffer-overflow in terminal handling

Problem:  When processing terminal output with many combining characters
          from supplementary planes (4-byte UTF-8), a heap-buffer
          overflow occurs. Additionally, the loop iterating over
          cell characters can read past the end of the vterm array
          (ehdgks0627, un3xploitable).
Solution: Use VTERM_MAX_CHARS_PER_CELL * 4 for ga_grow() to ensure
          sufficient space. Add a boundary check to the character
          loop to prevent index out-of-bounds access.

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-rvj2-jrf9-2phg

Signed-off-by: Christian Brabandt <cb@256bit.org>