Fix relative offsets when copying JMPZNZ

This was doing a plain copy of JMPZNZ, even though it encodes
offsets relative to the opline. As such, the offsets would be
relative to targe

Fix relative offsets when copying JMPZNZ

This was doing a plain copy of JMPZNZ, even though it encodes
offsets relative to the opline. As such, the offsets would be
relative to target, while they should be relative to opline.
Fix this by recomputing them.

Fixes oss-fuzz #39295.

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