unused-but-set-variable (proxyHost and proxyPort) build errors on Mozilla Try

When pushing tor-browser-31.6.0esr-4.5-1 to Mozilla Try, the build fails with the following error:

    /builds/slave/try-lx-d-000000000000000000000/build/src/netwerk/base/src/nsSocketTransport2.cpp:1083:21: error: variable 'proxyHost' set but not used [-Werror=unused-but-set-variable]
    /builds/slave/try-lx-d-000000000000000000000/build/src/netwerk/base/src/nsSocketTransport2.cpp:1084:17: error: variable 'proxyPort' set but not used [-Werror=unused-but-set-variable] 

https://treeherder.mozilla.org/#/jobs?repo=try&revision=f828bf8f28dd

I am attaching a patch to fix the issue.