Don't abort when merely checking if a non-map is a map

- When P_OpenMapData() is called by P_CheckMapData(), we don't actually
  care if any required lumps are missing. This just means it isn't a valid
  map, so don't abort with I_Error().
Don't abort for TEXTUREx lumps that define textures with no patches

- A texture defined in TEXTUREx without any patches isn't necessarily an
  error, so accept. This also means they shouldn't be used for determining
  if a TEXTURE directory belongs to Strife instead of Doom.
Merge branch 'maint'