patch 9.2.0817: crash when building a stacktrace during an autocommand

patch 9.2.0817: crash when building a stacktrace during an autocommand

Problem:  Vim can crash while building a stack trace when an autocommand
          is being triggered but has not matched a pattern yet, for
          example at the more prompt with 'verbose' set (stefanos82).
Solution: Handle the autocommand entry on the execution stack that does
          not have a script context yet (Hirohito Higashi).

fixes:  #20730
closes: #20732

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>