patch 9.0.0101: invalid memory access in diff mode with "dp" and undo

Problem:    Invalid memory access in diff mode with "dp" and undo.
Solution:   Make sure the line number does not go below one.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index dcacd55..41f7fe3 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ