Dungeongen: Fix out-of-voxelmanip access segfault

Dungeongen: Fix out-of-voxelmanip access segfault

My recent dungeon commit allowed stairs to be placed across the full
width of corridors, but some of the new node positions accessed were
missing checks for being within the voxelmanip, causing occasional
segfaults near dungeons with corridors wider than 1 node.

Add 'vm->m_area.contains(pos)' checks just before stair position
voxelmanip access. This allows an earlier check to be removed as it
is now redundant.