mesa: validate that sync objects were created by mesa

mesa: validate that sync objects were created by mesa

Previously, the user could send in a pointer that was not created
by mesa. When we dereferenced that pointer, there would be an
exception.

Now we keep a set of pointers and verify that the pointer
exists in that set before dereferencing it.

Note: This fixes several crashing gles3conform tests.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>