Backport #3810: bounds check iChanID before array access (release/3_12)

[mcfnord]

MY LLM WROTE:

Backports #3810's fix to release/3_12. A straight cherry-pick of 562cfd84 doesn't apply here: it calls MathUtils::InRange, which doesn't exist on this branch — it was added by #3741 and only reached its current half-open-interval semantics via #3812, neither of which is on release/3_12. Rather than backport that dependency chain too, this inlines the equivalent bounds check directly, with the same effect: iChanID arrives off the wire as an unvalidated value and this branch's ApplyNewConClientList had no check before indexing a 150-entry stack array with it.

Verified: -fsyntax-only clean, clang-format unchanged, full client build (qmake && make) compiles and links with no errors. Only src/audiomixerboard.cpp touched.

Fixes the same issue as #3810, for release/3_12.

[ann0see]

I don't like this inlining: we should ship the same code as on main.

[mcfnord]

MY LLM WROTE:

@ann0see Fair — pushed 57525a2b to ship the same code as main instead. Rather than inline the check, I backported just MathUtils::InRange (the template method itself, half-open-interval semantics from #3812) into class MathUtils in util.h. release/3_12's MathUtils class already exists, so this is a single self-contained method addition — no dependency on the rest of #3741/#3812's chat refactor. audiomixerboard.cpp's ApplyNewConClientList now calls MathUtils::InRange<int>( iChanID, 0, MAX_NUM_CHANNELS ), byte-for-byte identical to main's 562cfd84.

Verified: clang-format-14 --dry-run --Werror clean, full client build (qmake && make) compiles and links with no errors/warnings.

[pljones]

OK, that's small enough.

[ann0see]

I still prefer the exact same commits cherry picked.

[dingodoppelt]

I'd prefer cherry-picking the original commits. We didn't squash on merge so there are a few which could be squashed only for this backport.

[mcfnord]

MY LLM WROTE:

Force-pushed with actual cherry-picks instead of the hand-written equivalent.

29a13e49 squashes just the src/util.h hunks from the six main commits that shaped MathUtils::InRange into its current form — #3741's 562d6c66 + 4f9a532a, then #3812's 6d305cb7 + 65513e5b + fe98b1ad — applied in that order. I left out their server.cpp/server.h hunks: those belong to #3741's SendChatTextToConChannel/SendChatTextToAllConChannels refactor, which release/3_12 doesn't have and which is out of scope for a bounds-check backport. Squashing was necessary because several of those six commits are one-line follow-ups to each other (comment wording, style) — cherry-picking them individually would have produced intermediate states that don't compile standalone once the chat-refactor hunks are dropped.

f0180425 is git cherry-pick -x 562cfd84 unmodified — same commit, same author, (cherry picked from commit ...) trailer intact.

Net result is identical to the previous push, plus one line I'd missed: the // Returns true if value is in [lower, upper)... comment above InRange, which main has and my earlier hand-typed version didn't. Confirmed byte-for-byte against MathUtils::InRange on main.

Verified: clang-format-14 --dry-run --Werror clean, full client build (qmake && make) compiles and links Jamulus with no errors/warnings.