vim-patch:9.0.0101: invalid memory access in diff mode with "dp" and undo (#19568)

Problem:    Invalid memory access in diff mode with "dp" and undo.
Solution:   Make sure the line number does not go below one.
4e677b9c40

src/nvim/testdir/test_diffmode.vim

@@ -1471,5 +1471,19 @@ func Test_diff_manipulations()
 doobdeu
 
   set nodiff
+  %bwipe!
+endfunc
+
+" This was causing the line number in the diff block to go below one.
+" FIXME: somehow this causes a valgrind error when run directly but not when
+" run as a test.
+func Test_diff_put_and_undo()
+  set diff
+  next 0
+  split 00
+  sil! norm o0gguudpo0ggJuudp
+
+  bwipe!
+  bwipe!
   set nodiff
 endfunc