security/py-pycryptopp: Fix build with Clang 6

src/pycryptopp/publickey/ecdsamodule.cpp:94:16: error: cannot initialize return object of type 'int' with an rvalue of type 'nullptr_t'

return NULL;
       ^~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/py27-pycryptopp-0.7.1.869544967005693312591928092448767568728501330214.log