irc/minbif: Fix build with Clang 6

src/im/auth_pam.cpp:193:11: error: no matching function for call to 'pam_chauthtok'

retval = pam_chauthtok(pamh, NULL);
         ^~~~~~~~~~~~~

/usr/include/security/pam_appl.h:64:1: note: candidate function not viable: no known conversion from 'nullptr_t' to 'int' for 2nd argument
pam_chauthtok(pam_handle_t *_pamh,
^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p478276_s338342/logs/errors/minbif-1.0.5_10.log