patch 9.1.0812: Coverity warns about dereferencing NULL ptr

Problem:  Coverity warns about dereferencing NULL ptr
          in check_colorcolumn()
Solution: verify that wp is not null before accessing it

related: #15914

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/window.c b/src/window.c
index 5a8dc2e..7c8f4fb 100644
--- a/src/window.c
+++ b/src/window.c
@@ -7934,7 +7934,7 @@
     char_u *cc,		// when NULL: use "wp->w_p_cc"
     win_T *wp)		// when NULL: only parse "cc"
 {
-    char_u	*s;
+    char_u	*s = empty_option;
     int		tw;
     int		col;
     int		count = 0;
@@ -7947,7 +7947,7 @@
 
     if (cc != NULL)
 	s = cc;
-    else
+    else if (wp != NULL)
 	s = wp->w_p_cc;
 
     if (wp != NULL)