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.

src/diff.c

@@ -403,9 +403,9 @@ diff_mark_adjust_tp(
 		// 2. 3. 4. 5.: inserted/deleted lines touching this diff.
 		if (deleted > 0)
 		{
+		    off = 0;
 		    if (dp->df_lnum[idx] >= line1)
 		    {
-			off = dp->df_lnum[idx] - lnum_deleted;
 			if (last <= line2)
 			{
 			    // 4. delete all lines of diff
@@ -426,6 +426,7 @@ diff_mark_adjust_tp(
 			else
 			{
 			    // 5. delete lines at or just before top of diff
+			    off = dp->df_lnum[idx] - lnum_deleted;
 			    n = off;
 			    dp->df_count[idx] -= line2 - dp->df_lnum[idx] + 1;
 			    check_unchanged = TRUE;
@@ -434,7 +435,6 @@ diff_mark_adjust_tp(
 		    }
 		    else
 		    {
-			off = 0;
 			if (last < line2)
 			{
 			    // 2. delete at end of diff

src/testdir/test_diffmode.vim

@@ -1615,5 +1615,17 @@ 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

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5164,
 /**/
     5163,
 /**/