avm1: Function base clip defaults to `this` if clip no longer exists

When a function is defined, the base clip is stored in the function
object, and used when the function is called in SWFv6+. This
affects the target clip for GotoFrame and other actions.

However, if that base clip no longer exists when the function is
called, the base clip should default to the `this` display object.
Previously Ruffle would still use the previously unloaded base clip.

core/src/avm1/function.rs

@@ -305,7 +305,7 @@ impl<'gc> Executable<'gc> {
                 };
 
                 let max_recursion_depth = activation.context.avm1.max_recursion_depth();
-                let base_clip = if effective_ver > 5 {
+                let base_clip = if effective_ver > 5 && !af.base_clip.removed() {
                     af.base_clip
                 } else {
                     this.as_display_object()