Prevent null pointer exception for h->ai_canonname

Prevent null pointer exception for h->ai_canonname

The getaddrinfo call in line 54 sets &h to a struct addrinfo. If a
canonical name cannot be found for the node argument of getaddrinfo,
h->ai_canonname is set to NULL. In that case, the strchr call in line
58 can lead to segfault. This behavior was observed on a macos sierra
while the hostname was 192.168.1.3 (unfortunately this happens quite
often in macos).

The fix is simple, just check h->ai_canonname for the NULL value.