BUG: return immediately if memoryview cannot be non-native

late return triggers an assert with python3.4 in debug mode.
Closes gh-4138.
BUG: fix native-only long long check

Caused all long longs to be required native only in memoryviews.
NPY_SIZEOF_LONG_LONG never existed, replace documentation with existing
NPY_SIZEOF_LONGLONG and code with a sizeof(npy_longlong).