Make Racct macro slightly more gracious given RACCT_UNDEFINED

rctl_string_to_rule could previously index below the zeroth element of
racct_types via the macro. Maybe it shouldn't use the macro on
RACCT_UNDEFINED. But given every other RACCT_ definition is non-negative, it
seems pretty easy to foot-shoot this one without the check.

Reported by: Coverity
CID: 1305574
Sponsored by: EMC / Isilon Storage Division