This warning, IIUC, means that the compiler doesn't like it when it
sees a NULL check _after_ we've already dereferenced the
variable. In such cases, it considers itself free to eliminate the
NULL check.

There are a couple of tricky cases:

One was the case related to the fact that tor_addr_to_in6() can
return NULL if it gets a non-AF_INET6 address.  The fix was to
create a variant which asserts on the address type, and never
returns NULL.