patch 9.2.0513: [security]: memory safety issues in spellfile.c

patch 9.2.0513: [security]: memory safety issues in spellfile.c

Problem:  [security]: memory safety issues in spellfile.c
          (tacdm)
Solution: Add recursion limit to read_tree_node(), add length limit
          check in tree_count_words(), use alloc_clear() in
          spell_read_tree().

Github Security Advisory:
https://github.com/vim/vim/security/advisories/GHSA-3h95-3962-mmvf

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>