patch 9.2.0149: Vim9: segfault when unletting an imported variable

patch 9.2.0149: Vim9: segfault when unletting an imported variable

Problem:  do_unlet_var() unconditionally calls dictitem_remove() in its
          final else branch, but for imported items lp->ll_dict is NULL,
          causing a segfault (Peter Kenny)
Solution: Add a NULL check and return E1260 instead.

Affects :unlet at vim9script level and inside legacy :function.
The :def case already worked (handled in vim9cmds.c).

fixes:  #19637
closes: #19657

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>