patch 9.0.0535: closure gets wrong value in for loop with two loop variables

Problem:    Closure gets wrong value in for loop with two loop variables.
Solution:   Correctly compute the number of loop variables to clear.
diff --git a/src/structs.h b/src/structs.h
index e46446d..19dc550 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1630,7 +1630,7 @@
 typedef struct
 {
     int		fi_semicolon;	// TRUE if ending in '; var]'
-    int		fi_varcount;	// nr of variables in the list
+    int		fi_varcount;	// nr of variables in [] or zero
     int		fi_break_count;	// nr of line breaks encountered
     listwatch_T	fi_lw;		// keep an eye on the item used.
     list_T	*fi_list;	// list being used