Fix uninitialized variable warning.

src/core/SkLinearBitmapPipeline.cpp

Index: src/core/SkLinearBitmapPipeline.cpp
diff --git a/src/core/SkLinearBitmapPipeline.cpp b/src/core/SkLinearBitmapPipeline.cpp
index 1d08a8bd7c295efc21be311337bebe53261da47f..2090a1e436764f2c581b40d6b6f1444ec5d75fd7 100644
--- a/src/core/SkLinearBitmapPipeline.cpp
+++ b/src/core/SkLinearBitmapPipeline.cpp
@@ -177,7 +177,9 @@ void span_fallback(Span span, Stage* stage) {
     std::tie(start, length, count) = span;
     Sk4f xs{X(start)};
     Sk4f ys{Y(start)};
-    Sk4s fourDx;
+
+    // Initializing this is not needed, but some compilers can't figure this out.
+    Sk4s fourDx{0.0f};
     if (count > 1) {
         SkScalar dx = length / (count - 1);
         xs = xs + Sk4f{0.0f, 1.0f, 2.0f, 3.0f} * dx;