Py_INCREF(PylibMCExc_Error) before adding second reference to it

Otherwise, debug builds of Python dump core with

    python: Modules/gcmodule.c:379: visit_decref: Assertion
    `((gc)->gc.gc_refs >> (1)) != 0' failed.

(manually word-wrapped)
Merge pull request #164 from hfalcic/master

Fix #161