add 0->0 path to approx_powf()

The path involving approx_log2() and approx_pow2() does not produce 0.
And it's probably not a good idea to think about what approx_log2(0) is
anyway.

Change-Id: If5f48298c5bd5565ae808ebdfbd02649f4dd3046
Reviewed-on: https://skia-review.googlesource.com/85840
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/SkJumper_vectors.h

@@ -651,7 +651,8 @@ SI F approx_pow2(F x) {
 }
 
 SI F approx_powf(F x, F y) {
-    return approx_pow2(approx_log2(x) * y);
+    return if_then_else(x == 0, 0
+                              , approx_pow2(approx_log2(x) * y));
 }
 
 SI F from_half(U16 h) {