[TIR] Handle nullptr returned by FindEntryFunc

[Icemist]

The FindEntryFunc function can return a null pointer. In development I got this situation, which appears as a segfault.

[tvm-bot]

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• cc @Hzfengsy, @junrushao, @quic-sanirudh, @shingjan _{See #10317 for details}

_{Generated by tvm-bot}

[Icemist]

cc @masahi @echuraev

[junrushao]

Quick question: do you think it’s necessary to add a unittest?

[echuraev]

In general LGTM. But the same question as @junrushao have already asked. Can you add a unit test?

[echuraev]

LGTM. Thanks