fix gcc warning when CXXFLAGS contains '-Wextra'
fix gcc warning when CXXFLAGS contains '-Wextra'

json_value.cpp:179:26: warning: enumeral and non-enumeral type in conditional expression [enabled by default]

https://github.com/open-source-parsers/jsoncpp/pull/84

Streamlet/GoogleHelper,Streamlet/GoogleHelper,Streamlet/GoogleHelper,Streamlet/GoogleHelper,Streamlet/GoogleHelper