Doomsday Engine

Fixed|Client|Map|Clang: Crash in optimized build (undefined behavior)

During line owner sorting, LineOwner was returning references to null,
which is undefined behavior according to the C++ standard. The result
was that Clang's optimizer produced incorrect code.

http://stackoverflow.com/questions/2727834/c-standard-dereferencing-null-pointer-to-get-a-reference

IssueID #2052