Remove compile warning and possible error on zero size

sw/airborne/subsystems/navigation/OSAMNav.c

@@ -365,6 +365,9 @@ bool_t InitializePolygonSurvey(uint8_t EntryWP, uint8_t Size, float sw, float Or
 
 	CSurveyStatus = Init;
 	
+	if (Size == 0)
+	  return TRUE;
+
 	//Don't initialize if Polygon is too big or if the orientation is not between 0 and 90
 	if(Size <= PolygonSize && Orientation >= -90 && Orientation <= 90)
 	{