Avoid panic when concretizing symbolic bytes

[trufae]

Motivation

• Prevent a denial-of-service crash caused when concretizing symbolic memory bytes (via Solver::evalcon_to_u64) returns None and is unwrap()-ed during memory reads reachable from strtod/strtof simulators.

Description

• Replace the panic-prone solver.evalcon_to_u64(d).unwrap() with solver.evalcon_to_u64(d).unwrap_or(0) in Memory::read_bytes so unconcretizable bytes fall back to 0 instead of causing a panic, preserving prior fallback semantics while removing the crash vector.

Testing

• Ran cargo test -p radius2 --offline; the build/tests could not complete in this environment because boolector-sys's build script attempted to download Lingeling and curl failed with a CONNECT/403, so automated tests were not executable here.

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Codex Task