AP_Math: AP_GeodesicGrid: fix algorithm for null vector

If v is the null vector, then alpha * v is still the null vector for any alpha
as a real number. That means that the null vector doesn't cross any section.

libraries/AP_Math/AP_GeodesicGrid.cpp

@@ -347,8 +347,7 @@ int AP_GeodesicGrid::_triangle_index(const Vector3f& v,
     case 0:
         /* There are two possible cases when balance is 1:
          *
-         * 1) The vector v is the null vector. Arbitrarily return first
-         * triangle.
+         * 1) The vector v is the null vector, which doesn't cross any section.
          *
          * 2) One coefficient is zero, another is positive and yet another is
          * negative. Let a, b and c be the respective vertices for those
@@ -356,7 +355,7 @@ int AP_GeodesicGrid::_triangle_index(const Vector3f& v,
          * are also valid here.
          */
         if (zero_count == 3) {
-            return inclusive ? 0 : -1;
+            return -1;
         }
 
         if (!is_zero(w.x) && w.x < 0) {

libraries/AP_Math/AP_GeodesicGrid.h

@@ -121,8 +121,9 @@ public:
      * are checked is unspecified.  The default value for \p inclusive is
      * false.
      *
-     * @return The index of the section. The value -1 is returned if the
-     * section isn't found, which might happen when \p inclusive is false.
+     * @return The index of the section. The value -1 is returned if \p v is
+     * the null vector or the section isn't found, which might happen when \p
+     * inclusive is false.
      */
     int section(const Vector3f& v, const bool inclusive = false) const;
 
@@ -266,8 +267,8 @@ private:
      * #section() const.
      *
      * @return The index of the icosahedron triangle. The value -1 is returned
-     * if the triangle isn't found, which might happen when \p inclusive is
-     * false.
+     * if \p v is the null vector or the triangle isn't found, which might
+     * happen when \p inclusive is false.
      */
     int _from_neighbor_umbrella(int umbrella_index,
                                 const Vector3f& v,