Simplify flattening to just write enough to call the factory

src/core/SkImageFilter.cpp

Index: src/core/SkImageFilter.cpp
diff --git a/src/core/SkImageFilter.cpp b/src/core/SkImageFilter.cpp
index 4a9a22e63445bde4b0c6705403fe0dc08ee31805..4f2251eee186bd5f2018599eafa445b9d27d47ca 100644
--- a/src/core/SkImageFilter.cpp
+++ b/src/core/SkImageFilter.cpp
@@ -75,9 +75,9 @@ void SkImageFilter::Common::detachInputs(SkImageFilter** inputs) {
 }
 
 bool SkImageFilter::Common::unflatten(SkReadBuffer& buffer, int expectedCount) {
-    int count = buffer.readInt();
-    if (expectedCount < 0) {    // means the caller doesn't care how many
-        expectedCount = count;
+    const int count = buffer.readInt();
+    if (!buffer.validate(count >= 0)) {
+        return false;
     }
     if (!buffer.validate((count == expectedCount) && (count >= 0))) {
         return false;