Fix compilation errors with Clang
Description
Fix two unsigned comparisons to zero or more being always true.
I have a PR ready that fixes all the compilation errors I've encountered:
https://github.com/zeromq/libzmq/pull/607
Environment
FreeBSD clang version 3.2 (tags/RELEASE_32/final 170710) 20121221
Target: x86_64-unknown-freebsd9.1
Thread model: posix
