vim-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.
c101abff4c

src/nvim/testdir/test_diffmode.vim

@@ -1458,5 +1458,17 @@ func Test_diff_only()
   %bwipe!
 endfunc
 
+" This was causing invalid diff block values
+" FIXME: somehow this causes a valgrind error when run directly but not when
+" run as a test.
+func Test_diff_manipulations()
+  set diff
+  split 0
+  sil! norm R
+doobdeuR
+doobdeuR
+doobdeu
+
+  set nodiff
   %bwipe!
 endfunc