patch 9.1.0647: [security] use-after-free in tagstack_clear_entry

patch 9.1.0647: [security] use-after-free in tagstack_clear_entry

Problem:  [security] use-after-free in tagstack_clear_entry
          (Suyue Guo )
Solution: Instead of manually calling vim_free() on each of the tagstack
          entries, let's use tagstack_clear_entry(), which will
          also free the stack, but using the VIM_CLEAR macro,
          which prevents a use-after-free by setting those pointers
          to NULL

This addresses CVE-2024-41957

Github advisory:
https://github.com/vim/vim/security/advisories/GHSA-f9cr-gv85-hcr4

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