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