patch 9.1.1443: potential buffer underflow in insertchar()

patch 9.1.1443: potential buffer underflow in insertchar()

Problem:  potential buffer underflow in insertchar()
Solution: verify that end_len is larger than zero
          (jinyaoguo)

When parsing the end-comment leader, end_len can be zero if
copy_option_part() writes no characters. The existing check
unconditionally accessed lead_end[end_len-1], causing potential
underflow when end_len == 0.

This change adds an end_len > 0 guard to ensure we only index lead_end
if there is at least one character.

closes: #17476

Signed-off-by: jinyaoguo <guo846@purdue.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>