Check all PWADs selected to determine IWAD

Previously, if multiple PWADs were selected in the WAD launcher, with no
IWAD, the code would give up if one of the PWADs didn't contain any map
data. For example, with Requiem (http://doom.wikia.com/wiki/Requiem), if
you selected REQMUS.WAD and REQUIEM.WAD, it wouldn't load because
REQMUS.WAD contains no map data. Now every PWAD will be checked for map
data and the required IWAD will attempt to be loaded *before* the PWADs
are then merged.