patch 9.1.1818: possible crash when calculating topline in diff.c

Problem:  possible crash when calculating topline in diff.c
          (youngmith)
Solution: Check for pointer being Null before accessing it

fixes: #18437

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/diff.c

@@ -2226,6 +2226,7 @@ calculate_topfill_and_topline(
     diff_T	*thistopdiff = NULL;
     diff_T	*next_adjacent_blocks = NULL;
     int		virtual_lines_passed = 0;
+    int		curlinenum_to = 1;
 
     find_top_diff_block(&thistopdiff, &next_adjacent_blocks, fromidx, from_topline);
 
@@ -2253,7 +2254,8 @@ calculate_topfill_and_topline(
 
     // move the same amount of virtual lines in the target buffer to find the
     // cursor's line number
-    int curlinenum_to = thistopdiff->df_lnum[toidx];
+    if (thistopdiff != NULL)
+	curlinenum_to = thistopdiff->df_lnum[toidx];
 
     int virt_lines_left = virtual_lines_passed;
     curdif = thistopdiff;

src/testdir/test_crash.vim

@@ -143,6 +143,28 @@ func Test_crash1_2()
   call s:RunCommandAndWait(buf, args ..
     \ ' ; echo "crash 5: [OK]" >> '.. result)
 
+  let file = 'Xdiff'
+  let lines =<< trim END
+    diffs a
+    edit Xdiff
+    file b
+    exe "norm! \<C-w>\<C-w>"
+    exe "norm! \<C-w>\<C-w>"
+    exe "norm! \<C-w>\<C-w>"
+    exe "norm! \<C-w>\<C-w>"
+    exe "norm! \<C-w>\<C-w>"
+    exe "norm! \<C-w>\L"
+    exe "norm! \<C-j>oy\<C-j>"
+    edit Xdiff
+    sil!so
+  END
+  call writefile(lines, file, 'D')
+  let cmn_args = "%s -u NONE -i NONE -X -m -n -e -s -u %s -c ':qa!'"
+  let args = printf(cmn_args, vim, file)
+  call s:RunCommandAndWait(buf, args ..
+    \ ' && echo "crash 6: [OK]" >> '.. result)
+
+
   " clean up
   exe buf .. "bw!"
   exe "sp " .. result
@@ -152,6 +174,7 @@ func Test_crash1_2()
       \ 'crash 3: [OK]',
       \ 'crash 4: [OK]',
       \ 'crash 5: [OK]',
+      \ 'crash 6: [OK]',
       \ ]
 
   call assert_equal(expected, getline(1, '$'))

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1818,
 /**/
     1817,
 /**/