[wasm] Fix decoder for null data

If passing <nullptr, 0> to the decoder and trying to decode something,
it correctly detects the error and sets an error message, but still
returns true on ok(), and returns a valid result.
I triggered this error by passing a null Vector, returned by FindSection(), to
the decoder.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2410913002
Cr-Commit-Position: refs/heads/master@{#40204}

src/wasm/decoder.h

@@ -253,7 +253,7 @@ class Decoder {
   template <typename T>
   Result<T> toResult(T val) {
     Result<T> result;
-    if (error_pc_) {
+    if (failed()) {
       TRACE("Result error: %s\n", error_msg_.get());
       result.error_code = kError;
       result.start = start_;
@@ -279,8 +279,8 @@ class Decoder {
     error_msg_.reset();
   }
 
-  bool ok() const { return error_pc_ == nullptr; }
-  bool failed() const { return !!error_msg_; }
+  bool ok() const { return error_msg_ == nullptr; }
+  bool failed() const { return !ok(); }
   bool more() const { return pc_ < limit_; }
 
   const byte* start() { return start_; }

test/unittests/wasm/decoder-unittest.cc

@@ -671,6 +671,13 @@ TEST_F(DecoderTest, ReadI64v_extra_bits_positive) {
   EXPECT_FALSE(decoder.ok());
 }
 
+TEST_F(DecoderTest, FailOnNullData) {
+  decoder.Reset(nullptr, 0);
+  decoder.checkAvailable(1);
+  EXPECT_FALSE(decoder.ok());
+  EXPECT_FALSE(decoder.toResult(nullptr).ok());
+}
+
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8