netutils: make re class attribute and catch IndexError

These missing changes were initially agreed upon but then forgotten.
First, we move the valid name regex to the class-level such that it
won't be compiled for every invocation of GetIP() and we wrap the result
of getaddrinfo() into a try/except to catch a possible IndexError.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>