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/mapi/glapi/gen/gl_genexec.py

@@ -127,6 +127,7 @@ _mesa_initialize_exec_table(struct gl_context *ctx)
    exec = ctx->Exec;
    assert(exec != NULL);
 
+   assert(ctx->Version > 0);
 """