Fix GH-20733: heap buffer overflow in optimizer #20736
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Some (conditional) jump instructions can be the last one in the op_array, because they can jump to themselves. In those cases `i + 1` in the CFG builder can point to outside the op_array because `i` is already the last opline. To solve this we need to check against the end and prevent setting the successor out of bounds.
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In this example, the SCCP pass causes the JMPNZ instruction to be the last instruction in the op_array, and always jump to itself. In those cases
i + 1in the CFG builder can point to outside the op_array becauseiis already the last opline.To solve this we need to check against the end and prevent setting the successor out of bounds.
The proposed solution is not really pretty. Ideally SCCP can replace the JMPNZ opcode with a JMP opcode, but that's more risky to commit to a stable branch.