Update use of deprecated SkRuntimeEffect BuilderChild API

Change-Id: Ia9c4fe84b3bfdb09972958376f92289a458a4f32
diff --git a/libs/hwui/jni/Shader.cpp b/libs/hwui/jni/Shader.cpp
index 9018443..1b38920 100644
--- a/libs/hwui/jni/Shader.cpp
+++ b/libs/hwui/jni/Shader.cpp
@@ -295,7 +295,7 @@
     SkShader* shader = reinterpret_cast<SkShader*>(shaderHandle);
 
     SkRuntimeShaderBuilder::BuilderChild child = builder->child(name.c_str());
-    if (child.fIndex == -1) {
+    if (child.fChild == nullptr) {
         ThrowIAEFmt(env, "unable to find shader named %s", name.c_str());
         return;
     }