disallow GImpact shapes with zero triangles

src/main/java/com/jme3/bullet/collision/shapes/GImpactCollisionShape.java

@@ -70,25 +70,32 @@ public class GImpactCollisionShape extends CollisionShape {
     /**
      * Instantiate a shape based on the specified CompoundMesh and offset.
      *
-     * @param mesh the mesh on which to base the shape (not null, unaffected)
+     * @param mesh the mesh on which to base the shape (not null, must contain
+     * at least one triangle, unaffected)
      * @param offset the offset to add to the vertex positions (not null,
      * unaffected)
      */
     public GImpactCollisionShape(CompoundMesh mesh, Vector3f offset) {
+        Validate.require(mesh.countTriangles() > 0, "at least one triangle");
+
         this.nativeMesh = new CompoundMesh(mesh, offset);
         createShape();
     }
 
     /**
      * Instantiate a shape based on the specified native mesh(es).
      *
-     * @param submeshes the mesh(es) on which to base the shape (not null)
+     * @param submeshes the mesh(es) on which to base the shape (not null, must
+     * contain at least one triangle)
      */
     public GImpactCollisionShape(IndexedMesh... submeshes) {
         this.nativeMesh = new CompoundMesh();
         for (IndexedMesh submesh : submeshes) {
             nativeMesh.add(submesh);
         }
+        Validate.require(
+                nativeMesh.countTriangles() > 0, "at least one triangle");
+
         createShape();
     }
     // *************************************************************************
@@ -226,6 +233,9 @@ public void setScale(Vector3f scale) {
      * Instantiate the configured {@code btGImpactMeshShape}.
      */
     private void createShape() {
+        int numTriangles = nativeMesh.countTriangles();
+        assert numTriangles > 0 : numTriangles;
+
         long meshId = nativeMesh.nativeId();
         long shapeId = createShape(meshId);
         setNativeId(shapeId);

src/test/java/TestLibbulletjme.java

@@ -1191,14 +1191,15 @@ public void test012() {
 
         CollisionShape box = new BoxCollisionShape(0.1f, 0.2f, 0.3f);
         CollisionShape compound = new CompoundCollisionShape();
-        CollisionShape gimpact = new GImpactCollisionShape();
-
-        float[] hf = {0f, 0f, 0f, 0f};
-        CollisionShape heightfield = new HeightfieldCollisionShape(hf);
 
         int[] indexArray = {0, 0, 0};
         Vector3f[] positionArray = {new Vector3f(0f, 0f, 0f)};
         IndexedMesh indexedMesh = new IndexedMesh(positionArray, indexArray);
+        CollisionShape gimpact = new GImpactCollisionShape(indexedMesh);
+
+        float[] hf = {0f, 0f, 0f, 0f};
+        CollisionShape heightfield = new HeightfieldCollisionShape(hf);
+
         CollisionShape mesh = new MeshCollisionShape(true, indexedMesh);
 
         Plane pl = new Plane(Vector3f.UNIT_Y, 0f);