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>
diff --git a/src/testdir/test_scroll_opt.vim b/src/testdir/test_scroll_opt.vim
index d5d08a2..342d382 100644
--- a/src/testdir/test_scroll_opt.vim
+++ b/src/testdir/test_scroll_opt.vim
@@ -926,4 +926,23 @@
   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