Fix valgrind error in test fixture

Author: wesm

cpp/src/arrow/parquet/parquet-io-test.cc

@@ -120,7 +120,8 @@ class BufferReader : public io::RandomAccessFile {
 
   Status Read(int64_t nbytes, int64_t* bytes_read, uint8_t* buffer) override {
     memcpy(buffer, buffer_ + position_, nbytes);
-    position_ += nbytes;
+    *bytes_read = std::min(nbytes, buffer_size_ - position_);
+    position_ += *bytes_read;
     return Status::OK();
   }
 
@@ -172,7 +173,7 @@ TEST(TestParquetReadSource, Basics) {
 
   auto expected_buffer = std::make_shared<::parquet::Buffer>(data_buffer + 4, 7);
 
-  ASSERT_TRUE(expected_buffer->Equals(*pq_buffer));
+  ASSERT_TRUE(expected_buffer->Equals(*pq_buffer.get()));
 }
 
 }  // namespace parquet