patch 8.2.5164: invalid memory access after diff buffer manipulations

Problem:    Invalid memory access after diff buffer manipulations.
Solution:   Use zero offset when change removes all lines in a diff block.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index afa8f89..4c7aff5 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ