Fix warnings shown by cppcheck for wrt-commons

Fix warnings shown by cppcheck for wrt-commons

[Issue#] LINUXWRT-126
[Problem] Cppcheck appears some warnings for wrt-commons.
[Cause] Incorrect code.

[Solution] Make some changes(not 3rdparty).

[Verification] Run cppcheck on wrt-plugins-common (with
--suppress=incorrectStringBooleanError) or check cppcheck.txt generated by build
bot (ignore warnigs like "A boolean comparison with the string literal ...
is always true"). Ignore warnings about 3rdparty.

Change-Id: I150192e3f096307bf1a4a4cc2a12f6cf0c787d11