socks: Returns 0xF6 only if BAD_HOSTNAME

This commit modifies the behavior of `parse_extended_address` in such a way
that if it fails, it will always return a `BAD_HOSTNAME` value, which is then
used to return the 0xF6 extended error code.  This way, in any case that is
not a valid v2 address, we return the 0xF6 error code, which is the expected
behavior.

Signed-off-by: David Goulet <dgoulet@torproject.org>