ByteVector, List: Check indices if the standard library does (#1467)

If the standard library checks the index in std::vector::operator[]
(_GLIBCXX_ASSERTIONS, libc++ hardening modes), ByteVector::operator[]
and List::operator[] now check the index against their own size and
abort if it is out of range, as std::vector does.

The std::vector check alone is not enough: a ByteVector created by
mid() shares the buffer of the original byte vector, so an index past
the end of the smaller vector still passes it. List::operator[] walks
a std::list, which these checks do not cover.

Builds without these macros are unchanged; a GCC Release build of
libtag produces identical code. The Ubuntu CI job now builds with
-D_GLIBCXX_ASSERTIONS so that the tests run with the checks enabled.

.github/workflows/build.yml

@@ -11,6 +11,8 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest, windows-2025, ubuntu-26.04]
         include:
+          - os: ubuntu-latest
+            cmake_extra_args: '-DCMAKE_CXX_FLAGS="-D_GLIBCXX_ASSERTIONS"'
           - os: windows-latest
             cmake_extra_args: '-DCMAKE_TOOLCHAIN_FILE="$env:VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake"'
           - os: windows-2025