setup: dynamically set size of iwad_labels array

With the addition of the Freedoom IWADs, the number of IWADs supported
by chocolate-doom has been raised to 10. However, the iwad_labels[]
array only holds place for up to 8 pointers. Incidently, I have all 10
IWADs installed and trying to warp into a game from
chocolate-doom-setup leads to an out-of-bounds access of this array
and so the application crashes with a segmentation fault.

Instead of increasing the array size to 10, which will bite us next
time, I decided to set its size dynamically as soon as the number of
IWADs of known.