gvariant: Check offset table doesn’t fall outside variant bounds and speed up text parsing

Fixes for two more bugs found by oss-fuzz.

See the commit messages and linked issues for details.

These commits have been reviewed under embargo on the issues.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #2840 (closed), #2841 (closed) oss-fuzz#54302, #54314

Closes #2840 (closed), #2841 (closed)