mesa: Don't leak gl_context::BeginEnd at context destruction

The other dispatch tables (Exec and Save) are freed, but BeginEnd is
never freed.  This was found by inspection why investigating the leak of
shared state in _mesa_initialize_context.

NOTE: This is a candidate for stable branches

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/context.c

@@ -1181,6 +1181,7 @@ _mesa_free_context_data( struct gl_context *ctx )
    _mesa_reference_buffer_object(ctx, &ctx->Array.ArrayBufferObj, NULL);
 
    /* free dispatch tables */
+   free(ctx->BeginEnd);
    free(ctx->Exec);
    free(ctx->Save);