#4 Fix regression in automatic_face_movement_max_rotation_per_sec

Values <= 0 should make the yaw change instant. This worked in 0.4.16 but was broken in https://notabug.org/pgimeno/minetest/commit/089f594582862d041a5bf3fa0f41d637248e31de. Per bug report by oil_boi_minetest on IRC.