Fix always introducing the unexplored label. Fixes #559.

[tquatmann]

./bin/storm --prism ../resources/examples/testfiles/dtmc/die.pm --state-limit 5

now introduces the unexplored label

-------------------------------------------------------------- 
Model type: 	DTMC (sparse)
States: 	5
Transitions: 	7
Reward Models:  none
State Labels: 	3 labels
   * unexplored -> 3 item(s)
   * deadlock -> 0 item(s)
   * init -> 1 item(s)
Choice Labels: 	none
-------------------------------------------------------------- 

while the invocation without a state limit does not:

 ./bin/storm --prism ../resources/examples/testfiles/dtmc/die.pm                

-------------------------------------------------------------- 
Model type: 	DTMC (sparse)
States: 	13
Transitions: 	20
Reward Models:  none
State Labels: 	2 labels
   * deadlock -> 0 item(s)
   * init -> 1 item(s)
Choice Labels: 	none
-------------------------------------------------------------- 

[volkm]

Thanks for the quick fix! LGTM