[Positioning] Direction: 0 is valid to construct

Accept 0 as a "valid" value for the constructor and operator=()
(constructing a Direction with 0 value was possible any way, using the
default constructor).
This fixes the reception of requests on UDP.

owlps-positioning/src/direction.cc

@@ -21,7 +21,7 @@ Direction::Direction(const char source):
 
 inline void Direction::assert_valid() const
 {
-  if (! is_valid())
+  if (! (is_valid() || direction == 0))
     throw bad_direction(direction) ;
 }