map: fix coverity issue 205684

ip4_map_get_domain returns d0 = 0 in case of no domain,
and sets error MAP_ERROR_NO_DOMAIN. Rearrange the clauses
in the logical check that relies on dereferencing d0
such that boolean shortcutting takes care of not
referencing the null pointer.

Change-Id: I378adc120836ea920fc530808ac3c1f98ab8d593
Type: fix
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>