fix infinite loop in debug_regexp

[RasmusWL]

On issue #13 I was seeing an infinite loop for debug_regexp I wont guarantee this makes the code 100% bugfree, but at least it solves my case.

This is what happened:

bounds: (0, 2542)
half: 1271
bounds: (0, 1270)
half: 635
bounds: (0, 634)
half: 317
bounds: (0, 316)
half: 158
bounds: (0, 157)
half: 78
bounds: (0, 77)
half: 38
bounds: (0, 37)
half: 18
bounds: (0, 17)
half: 8
bounds: (8, 17)
half: 12
bounds: (12, 17)
half: 14
bounds: (14, 17)
half: 15
bounds: (15, 17)
half: 16
bounds: (16, 17)
half: 16
bounds: (16, 17)
half: 16
bounds: (16, 17)
half: 16
bounds: (16, 17)
half: 16
...

[phillipberndt]

Thanks!