check unpremul scale directly against infinity

There are non-zero values of a that make infinite 1.0f/a.
Let's just check for the real thing we care about, that
scale is finite.

Bug: skia:7123

Change-Id: If97574c9f3f2f0b73c749d0bea9aa19e6114f4d1
Reviewed-on: https://skia-review.googlesource.com/58460
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/SkJumper_stages.cpp

@@ -572,7 +572,8 @@ STAGE(premul_dst, Ctx::None) {
     db = db * da;
 }
 STAGE(unpremul, Ctx::None) {
-    auto scale = if_then_else(a == 0, 0, 1.0f / a);
+    float inf = bit_cast<float>(0x7f800000);
+    auto scale = if_then_else(1.0f/a < inf, 1.0f/a, 0);
     r *= scale;
     g *= scale;
     b *= scale;