Fix T42367: Spline-ik offset evaluating curve

Referenced by issue #42367, Spline IK leaves a gap at the start of the bone chain

source/blender/blenkernel/intern/anim.c

@@ -627,6 +627,9 @@ int where_on_path(Object *ob, float ctime, float vec[4], float dir[3], float qua
 	if (!bl->nr) return 0;
 	if (bl->poly > -1) cycl = 1;
 
+	/* values below zero for non-cyclic curves give strange results */
+	BLI_assert(cycl || ctime >= 0.0f);
+
 	ctime *= (path->len - 1);
 	
 	s1 = (int)floor(ctime);

source/blender/blenkernel/intern/armature.c

@@ -1940,6 +1940,9 @@ static void splineik_init_tree_from_pchan(Scene *scene, Object *UNUSED(ob), bPos
 			}
 		}
 
+		/* disallow negative values (happens with float precision) */
+		CLAMP_MIN(ikData->points[segcount], 0.0f);
+
 		/* spline has now been bound */
 		ikData->flag |= CONSTRAINT_SPLINEIK_BOUND;
 	}