typo fix.
Ensure returning a EX_* error code in exec_audit.

Fixed a codepath where exec_audit could return EPKG_ERROR, since it
should only use sysexit error codes.
Always check the return value of pkg_new().

pkg_new() can fail when malloc(3) fails (ENOMEM) and we should always
ensure that the call was successfull and act acordingly on error.