patch 8.0.0287: debug mode: cannot access function arguments

Problem:    Cannot access the arguments of the current function in debug mode.
            (Luc Hermitte)
Solution:   use get_funccal(). (Lemonboy, closes #1432, closes #1352)
diff --git a/src/userfunc.c b/src/userfunc.c
index 6a6cc06..516ab47 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3556,7 +3556,7 @@
 {
     if (current_funccal == NULL)
 	return NULL;
-    return &current_funccal->l_avars_var;
+    return &get_funccal()->l_avars_var;
 }
 
 /*