patch 8.2.1239: "maxwidth" in 'completepopup' not obeyed

Problem:    "maxwidth" in 'completepopup' not obeyed. (Jay Sitter)
Solution:   Add separate field for value from option. (closes #6470)
diff --git a/src/structs.h b/src/structs.h
index 68a9ba9..ba9c98d 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3302,6 +3302,7 @@
     int		w_minwidth;	    // "minwidth" for popup window
     int		w_maxheight;	    // "maxheight" for popup window
     int		w_maxwidth;	    // "maxwidth" for popup window
+    int		w_maxwidth_opt;	    // maxwidth from option
     int		w_wantline;	    // "line" for popup window
     int		w_wantcol;	    // "col" for popup window
     int		w_firstline;	    // "firstline" for popup window