patch 8.2.1154: Vim9: crash when using imported function

Problem:    Vim9: crash when using imported function.
Solution:   Check for a function type.  Set the script context when calling a
            function. (closes #6412)
diff --git a/src/scriptfile.c b/src/scriptfile.c
index 9ffc66c..ce4bafa 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -68,7 +68,7 @@
 /*
  * Add a user function to the execution stack.
  */
-    void
+    estack_T *
 estack_push_ufunc(ufunc_T *ufunc, long lnum)
 {
     estack_T *entry = estack_push(ETYPE_UFUNC,
@@ -76,6 +76,7 @@
 				  ? ufunc->uf_name_exp : ufunc->uf_name, lnum);
     if (entry != NULL)
 	entry->es_info.ufunc = ufunc;
+    return entry;
 }
 
 /*
@@ -97,13 +98,15 @@
 #endif
 
 /*
- * Take an item off of the execution stack.
+ * Take an item off of the execution stack and return it.
  */
-    void
+    estack_T *
 estack_pop(void)
 {
-    if (exestack.ga_len > 1)
-	--exestack.ga_len;
+    if (exestack.ga_len == 0)
+	return NULL;
+    --exestack.ga_len;
+    return ((estack_T *)exestack.ga_data) + exestack.ga_len;
 }
 
 /*