BUG/MEDIUM: maps: only try to parse the default value when it's present

BUG/MEDIUM: maps: only try to parse the default value when it's present

Maps returning an IP address (e.g. map_str_ip) support an optional
default value which must be parsed. Unfortunately the parsing code does
not check for this argument's existence and uncondtionally tries to
resolve the argument whenever the output is of type address, resulting
in segfaults at parsing time when no such argument is provided. This
patch adds the appropriate check.

This fix may be backported as far as 1.6.