Standard 80C51 is designed to withstand such 'shortcuts'. So, nothing bad will happen.

But never make such a shortcut, when ports are emitting address bytes or data bytes, when addressing external memory! Then, strong pullups are activated much longer than two oscillator periods and damage will occur.

Also, never directly apply +Vcc to any port pin, unless you are sure never to fabricate low level state at this pin!!

Keep in mind, that newer derivatives can show different behaviour. Sometimes port outputs can be programmed to show push-pull performance. Then, such a shortcut would surely result in a damage!

Kai