Fix for [#30438] x=zero not accepted for a bone tail.

In fact, problem was in vec_roll_to_mat3(), which has to detect when the bone is aligned with its Y axis, using a threshold. This one have been raised to quite a high value due to bug [#23954], then lowered a bit due to [#27675] (which is imho in fine the same problem as 30438).

Reset it to its org value (very low 1e-13), as testing file given with firt bug did not show any problem anymore... So now, instead of 1/1000 of bone length from Y axis, we have about 3.25*10-7... Only (hardly) noticeable at max zoom level in 3D view.

source/blender/blenkernel/intern/armature.c

@@ -1513,8 +1513,11 @@ void vec_roll_to_mat3(const float vec[3], const float roll, float mat[][3])
 	 *
 	 * was 0.00001, causes bug [#27675], with 0.00000495,
 	 * so a value inbetween these is needed.
+	 *
+	 * was 0.000001, causes bug [#30438] (which is same as [#27675, imho).
+	 * Reseting it to org value seems to cause no more [#23954]...
 	 */
-	if (dot_v3v3(axis,axis) > 0.000001f) {
+	if (dot_v3v3(axis,axis) > 1.0e-13f) {
 		/* if nor is *not* a multiple of target ... */
 		normalize_v3(axis);