Fix crash in AudioUnit backend due to off-by-one error in parameter syncing

[xARSENICx]

Bug Description

Fixes a crash on macOS when using AudioUnit effects in a Debug build.
The syncParameters() method had an off-by-one error when growing the m_lastValues list.
When i=0 and the list is empty:

• The check if (m_lastValues.size() < i) (0 < 0) evaluated to false.
• The list did not grow.
• Accessing m_lastValues[i] caused an index-out-of-bounds assertion failure.

Reproduction Steps

1. Build Mixxx on macOS in Debug mode (-DCMAKE_BUILD_TYPE=Debug).
   • Note: The crash relies on Qt's container bounds assertions, which are often disabled in Release builds.
2. Start Mixxx with a fresh configuration (or ensure an AudioUnit effect is instantiated).
3. Load an AudioUnit effect into an effect chain (or trigger any action that instantiates AudioUnitEffectProcessor).
4. Result: Mixxx crashes immediately with ASSERT failure in QList::operator[]: "index out of range".

The Fix

Changed the condition to if (m_lastValues.size() <= i) to ensure the list grows correctly when size == index.

Verification

Run this command on a Mac to verify the crash (before the fix) and the solution (after the fix):

# 1. Setup Debug Build
cmake -S . -B build_debug -G Ninja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_TOOLCHAIN_FILE=...

# 2. Build
cmake --build build_debug -j10

# 3. Run (This will crash without the fix)
./build_debug/mixxx

[daschuer]

Nice catch. This 1 line PR was probably more work than others with 1000 lines. Thank you for that.

Can you please rebase it to 2.5? The issue is already present there.

"git rebase --onto=2.5 HEAD~1"

[xARSENICx]

This 1 line PR was probably more work than others with 1000 lines.

It did take me hours of digging into the codebase after struggling to continue my work on other features due to crashes. I hope others benefit from this and save time. 😄

[daschuer]

LGTM, Thank you.