gallium: add another value check to util_fast_pow()

Fixes glitches seen in morph3d demo.

src/gallium/auxiliary/util/u_math.h

@@ -272,8 +272,10 @@ util_fast_log2(float val)
 static INLINE float
 util_fast_pow(float x, float y)
 {
-   /* XXX this test may need adjustment */
-   if (y >= 3.0 && -0.02f <= x && x <= 0.02f)
+   /* XXX these tests may need adjustment */
+   if (y >= 3.0f && (-0.02f <= x && x <= 0.02f))
+      return 0.0f;
+   if (y >= 50.0f && (-0.9f <= x && x <= 0.9f))
       return 0.0f;
    return util_fast_exp2(util_fast_log2(x) * y);
 }