Designing state machine for Jk FF Counter
You can use two methods to solve this.


Use a self starting counter.
In this case the invalid states are also included while preparing the truth table.
The next state for an invalid state can be any valid state.Usually it is taken as the nearest 
valid state.
Non self starting counter in which you don't include the invalid states.While drawing the k-map, cells corresponding to invalid states are considered don't care.