Fix acosx condition (#17846)

Marlin/src/module/planner.cpp

                       t = neg * junction_cos_theta;
 
           // If angle is greater than 135 degrees (octagon), find speed for approximate arc
-          if (t < -0.7071067812f) {
+          if (t > 0.7071067812f) {
 
             #if ENABLED(JD_USE_MATH_ACOS)
 
                 4.85301876f, 6.77019119f, 9.50873947f, 13.4009094f, 18.9188652f,
                 26.7320709f, 37.7884521f, 53.4292908f, 75.5522461f,  0.0f };
 
-              const int16_t idx = (t == 0.0f) ? 0 : __builtin_clz(int16_t((1.0f - t) * jd_lut_tll)) - jd_lut_tll0;
+              const int16_t idx = (t == 0.0f) ? 0 : __builtin_clz(uint16_t((1.0f - t) * jd_lut_tll)) - jd_lut_tll0;
 
               float junction_theta = t * pgm_read_float(&jd_lut_k[idx]) + pgm_read_float(&jd_lut_b[idx]);
               if (neg > 0) junction_theta = RADIANS(180) - junction_theta;