patch 8.2.4322: Vim9: crash when using funcref with closure

Problem:    Vim9: crash when using funcref with closure.
Solution:   Keep a reference to the funcref that has the outer context.
            (closes #9716)
diff --git a/src/structs.h b/src/structs.h
index ecab354..1e759f5 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2051,6 +2051,9 @@
     // For a compiled closure: the arguments and local variables scope
     outer_T	pt_outer;
 
+    // For a partial of a partial: use pt_outer values of this partial.
+    partial_T	*pt_outer_partial;
+
     funcstack_T	*pt_funcstack;	// copy of stack, used after context
 				// function returns