Fix ARM panic when a code section has no mapping symbols

[SAY-5]

get_mapping_symbols inserts an entry into disasm_modes for every code section, even when no $a/$t/$d mapping symbol points into it, so scan_instructions_internal got an empty slice from disasm_modes.get(..) instead of None and skipped the fallback_mappings path, then indexed the empty slice and panicked. This treats the empty case the same as a missing entry so it falls back to ARM mode. Fixes #352.

[SAY-5]

Bumping this one. It's a small bounds guard for the ARM no-mapping-symbols panic, still applies cleanly on main. Happy to adjust if you'd like anything changed.

[LagoLunatic]

Bumping this one. It's a small bounds guard for the ARM no-mapping-symbols panic, still applies cleanly on main. Happy to adjust if you'd like anything changed.

Is there a reason we need 3 separate PRs for a one line crash fix?
#359 (mine)
#361 (yours)
#365 (yours)

[SAY-5]

You're right, that's redundant, sorry for the noise. Your #359 fixes it, so I'll close both of mine. If the regression test and fixture from #365 are useful I'm happy to send them as a separate PR against your fix, otherwise feel free to ignore.