mesa: assert if driver did not compute the version

Make sure drivers initialize the version before:
 * _mesa_initialize_exec_table is called
 * _mesa_initialize_exec_table_vbo is called
 * A context is made current

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/context.c

@@ -1461,7 +1461,7 @@ _mesa_make_current( struct gl_context *newCtx,
       }
 
       if (newCtx->FirstTimeCurrent) {
-         _mesa_compute_version(newCtx);
+         assert(newCtx->Version > 0);
 
          newCtx->Extensions.String = _mesa_make_extension_string(newCtx);