mesa: fix double-freeing of dispatch tables inside glBegin/End.

mesa: fix double-freeing of dispatch tables inside glBegin/End.

We allocate dispatch tables for BeginEnd and OutsideBeginEnd.  But
when we destroy the context we were freeing the BeginEnd and Exec
tables.  If Exec==BeginEnd we did a double-free.  This would happen
if the context was destroyed while inside a glBegin/End pair.  Now
free the BeginEnd and OutsideBeginEnd pointers.

Cc: "10.1", "10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>