[BUG] Arrays of external write-buffered components crash #208

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Describe the bug
Arrays of external components with write-buffered registers crash the exporter.

Details like these can be helpful:
peakrdl-regblock v1.3.1

Reproduced with

 external myreg ext_reg_array[2];

added to the read buffer/write buffer test RDL files.

Error message:

    @property
    def current_loop(self) -> Body:
>       return self._stack[-1]
E       IndexError: list index out of range

../src/peakrdl_regblock/forloop_generator.py:44: IndexError

Additional context
Bug introduced in #192. Specifically, this method was added to the read buffering/write buffering for loop implementations:

    def enter_AddressableComponent(self, node: AddressableNode) -> Optional[WalkerAction]:
        if node.external:
            return WalkerAction.SkipDescendants
        return super().enter_AddressableComponent(node)

Note that the parent's enter_AddressableComponent is not called if the node is external. If the node is an array, that means the array's dimensions are not "pushed" into the loop generator. However, the default exit_AddressableComponent method still tries to "pop" those dimensions from the generator.

PR incoming.