mesa: free debug messages when destroying the debug state

Fixes: 04a8baad "mesa: refactor _mesa_PopDebugGroup and _mesa_free_errors_data"

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98281
(cherry picked from commit a63e69f5)