Avoid an unnecessary allocation in recvMessage

I was looking at a potential issue reported [here](https://groups.google.com/forum/#!topic/golang-nuts/0HW4T81tw1Q).  With a [slightly altered](https://gist.github.com/johto/f7aab06c96486230c57d) benchmark I was able to measure a performance gain of ~10% by avoiding an unnecessary allocation in recvMessage.  Disassemblies of the two binaries [here](https://gist.github.com/johto/8654f9a9d36cc274c569).

The other allocation which seemed to cause problems was the readBuf allocation at the end of recvMessage, but getting rid of that is a slightly bigger patch.  Any objections to this one in the meanwhile?