clang: fix warning by adding assert that shows that a variable is used

Change-Id: I26e0ba9f1ae9d43c9a14c42c4225746782dc4d66
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/134
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/jtag/tcl.c

@@ -166,6 +166,8 @@ static int Jim_Command_drscan(Jim_Interp *interp, int argc, Jim_Obj *const *args
 		}
 	} /* validate args */
 
+	assert(e == JIM_OK);
+
 	tap = jtag_tap_by_jim_obj(interp, args[1]);
 	if (tap == NULL) {
 		return JIM_ERR;