workaround for undefined strncpy behavior when src and dest are the same

Just tried building on Mac OSX 10.9 Mavericks and hit a crash while loading a map as Apple's libc implementation is now asserting if src == dest for strncpy.

It seems this has always been undefined behavior, but it worked I suppose.

I found another thread of someone else having the same issue:

http://postgresql.1045698.n5.nabble.com/OSX-doesn-t-accept-identical-source-target-for-strcpy-anymore-td5776101.html