What would happen if an Indian passenger aeroplane entered Pakistan due to bad weather?

 There is something called international civil aviation law, and both India and Pakistan are signatories on it. A couple of important points are:

1. Any civil aircraft can fly over any other country.
2. Any aircraft can take off from its home country land in a foreign country and vice versa.
3. Any aircraft can fly from a foreign country to another country.

In the case of an emergency, the plane will be allowed to land, although the passengers might not be allowed to deboard, or maybe not allowed outside the airport premises.

Athough there are many other laws, it basically states that civil aircrafts can fly over whichever country they wish to, by paying a certain fee.

Many Indian flights fly over Pakistan regularly.

Here is a route of the flights to Dubai from different cities of India.

In fact, aircraft carriers don't want to have to fly over pakistan, and use the Arabian Sea to go to gulf countries, since they'd save on fuel, time and airspace fees. They currently have to go via Pakistan because the air Force has restricted access to certain airspace over Gujarat .