Fix issues found by -Wextra and start compiling with it.

Everything it pointed out was harmless, but being extra pedantic does
not hurt.  -Wno-missing-field-initializers is also added to the
compiler flags because zero initializing a std::array with empty braces
should not be a compiler warning.
Fix easylogging++ not building on Android with -Werror.

A minor matter of ifdefing out code that read device properties that
obfsclient will NEVER call for obvious reasons.
Fix a potential bug found by clang's scan-build.

With all current uses of Ctr, the conditional I was missing is never
triggered since the paramenters are guaranteed to be sane, but this is
not guaranteed to be true in the future.

scan-build also points out a bunch of things in easylogging++ and gtest
but they don't look to be real problems.