ARROW-7354: [C++] Fix crash in test-io-hdfs

Closes #6000 from pitrou/ARROW-7354-hdfs-test-fix and squashes the following commits:

c187e3abb <Antoine Pitrou> ARROW-7354:  Fix crash in test-io-hdfs

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
diff --git a/cpp/src/arrow/io/hdfs_test.cc b/cpp/src/arrow/io/hdfs_test.cc
index 71e51a3..f3613f1 100644
--- a/cpp/src/arrow/io/hdfs_test.cc
+++ b/cpp/src/arrow/io/hdfs_test.cc
@@ -466,12 +466,12 @@
     for (int i = 0; i < niter; ++i) {
       std::shared_ptr<Buffer> buffer;
       if (i % 2 == 0) {
-        ASSERT_OK(file->ReadAt(3, 3, &buffer));
+        ASSERT_OK_AND_ASSIGN(buffer, file->ReadAt(3, 3));
         if (0 == memcmp(data.c_str() + 3, buffer->data(), 3)) {
           correct_count += 1;
         }
       } else {
-        ASSERT_OK(file->ReadAt(0, 4, &buffer));
+        ASSERT_OK_AND_ASSIGN(buffer, file->ReadAt(0, 4));
         if (0 == memcmp(data.c_str() + 0, buffer->data(), 4)) {
           correct_count += 1;
         }