patch 9.0.2107: [security]: FPE in adjust_plines_for_skipcol

Problem:  [security]: FPE in adjust_plines_for_skipcol
Solution: don't divide by zero, return zero

Prevent a floating point exception when calculating w_skipcol (which can
happen with a small window when the number option is set and cpo+=n).

Add a test to verify

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

src/move.c

@@ -45,8 +45,9 @@ adjust_plines_for_skipcol(win_T *wp)
 	return 0;
 
     int width = wp->w_width - win_col_off(wp);
-    if (wp->w_skipcol >= width)
-	return (wp->w_skipcol - width) / (width + win_col_off2(wp)) + 1;
+    int w2 = width + win_col_off2(wp);
+    if (wp->w_skipcol >= width && w2 > 0)
+	return (wp->w_skipcol - width) / w2 + 1;
 
     return 0;
 }

src/testdir/test_scroll_opt.vim

@@ -926,4 +926,23 @@ func Test_smoothscroll_cursor_top()
   call StopVimInTerminal(buf)
 endfunc
 
+" Division by zero, shouldn't crash
+func Test_smoothscroll_crash()
+  CheckScreendump
+
+  let lines =<< trim END
+      20 new
+      vsp
+      put =repeat('aaaa', 20)
+      set nu fdc=1  smoothscroll cpo+=n
+      vert resize 0
+      exe "norm! 0\<c-e>"
+  END
+  call writefile(lines, 'XSmoothScrollCrash', 'D')
+  let buf = RunVimInTerminal('-u NONE -S XSmoothScrollCrash', #{rows: 12, cols:40})
+  call term_sendkeys(buf, "2\<C-E>\<C-L>")
+
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2107,
 /**/
     2106,
 /**/