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/move.c b/src/move.c
index ce06dc3..fbb352a 100644
--- a/src/move.c
+++ b/src/move.c
@@ -45,8 +45,9 @@
 	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;
 }