HDFS-15232. Fix libhdfspp test failures with GCC 7.

[aajisaka]

JIRA: https://issues.apache.org/jira/browse/HDFS-15232

[smengcl]

lgtm. +1

[aajisaka]

The precommit job report:

https://builds.apache.org/job/hadoop-multibranch/view/change-requests/job/PR-1906/lastBuild/artifact/out/gitcommentfile.1

---

💔 -1 overall

Vote	Subsystem	Runtime	Comment
0	reexec	80	Docker mode activated.
		_ Prechecks _
+1	dupname	0	No case conflicting files found.
+1	@author	0	The patch does not contain any @author tags.
-1	test4tests	0	The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
		_ trunk Compile Tests _
+1	mvninstall	1306	trunk passed
+1	compile	109	trunk passed
+1	mvnsite	21	trunk passed
+1	shadedclient	2368	branch has no errors when building and testing our client artifacts.
		_ Patch Compile Tests _
+1	mvninstall	14	the patch passed
+1	compile	105	the patch passed
+1	cc	105	the patch passed
+1	golang	105	the patch passed
+1	javac	105	the patch passed
+1	mvnsite	14	the patch passed
+1	whitespace	0	The patch has no whitespace issues.
+1	shadedclient	913	patch has no errors when building and testing our client artifacts.
		_ Other Tests _
+1	unit	420	hadoop-hdfs-native-client in the patch passed.
+1	asflicense	30	The patch does not generate ASF License warnings.
		4095

Subsystem	Report/Notes
Docker	Client=19.03.8 Server=19.03.8 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-1906/1/artifact/out/Dockerfile
GITHUB PR	#1906
Optional Tests	dupname asflicense compile cc mvnsite javac unit golang
uname	Linux 175de3261623 4.15.0-74-generic #84-Ubuntu SMP Thu Dec 19 08:06:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / 62c8015
Default Java	1.8.0_242
Test Results	https://builds.apache.org/job/hadoop-multibranch/job/PR-1906/1/testReport/
Max. process+thread count	307 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs-native-client U: hadoop-hdfs-project/hadoop-hdfs-native-client
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-1906/1/console
versions	git=2.17.1 maven=3.6.0�[m
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[aajisaka]

Thank you, @smengcl

[smengcl]

Thanks for the patch, @aajisaka !