Fix getaddrinfo test to use random mapping ids

This seems to have been intended but it's just using the loop index. In
fact the only thing the random index is used for is an assert to check
that it is a valid index.

Also stops the test segfaulting if num_queries > num_hosts

Change-Id: I8ed5547145c6a8c9ce0adcb65f3777404d186af8