DiagonalFusion::aggregate_operations can crash in some circumstances

[aromanro]

Informations

• Qiskit Aer version: Latest development version
• Python version: Not relevant as qiskit aer was used from C++ when noticing it
• Operating system: Windows 11

What is the current behavior?

I was testing the MPS simulator with some randomly generated circuits... with some I've got crashes.
The call chain (a part of it) of this reported crash is AerState::apply_measure -> AerState::flush_ops -> AerState::transpile_ops -> Fusion::optimize_circuit -> ... -> DiagonalFusion::get_next_diagonal_end -> DiagonalFusion::is_diagonal_op

The last call is with an invalid op. The problem occurs because DiagonalFusion::get_next_diagonal_end gets an invalid from parameter.
The ops list had 102 size and next_diagonal_start was 102.

Steps to reproduce the problem

This would be probably hard to reproduce, I was generating some random circuits with more than 100 ops to test some things and maybe one in 100 circuits crashed (it's just an order of magnitude guesstimation), probably related with this (I also found this #2286 during the tests).

What is the expected behavior?

No crash.

Suggested solutions

DiagonalFusion::get_next_diagonal_end

qiskit-aer/src/transpile/fusion.hpp

Line 578 in 582407a

int DiagonalFusion::get_next_diagonal_end(

could have this check added right at the beginning:

  if (ops.size() <= from)
    return -1;