Fix binja backend stack string detection

[xusheng6]

This fixes a bug in the binja stack string detection code.

I was writing the code with a newer version of binja. It does not work with the stable headless used for the CI.

Checklist

• No CHANGELOG update needed

• No new tests needed

• No documentation update needed

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[xusheng6]

I will sign the CLA after discussing with the company

I also notice a rule linter failure for forwarded export. Although BN currently does not support forwarded export, I think this is not related, right?

[xusheng6]

I am closing this and will create a new one