Fix logically dead code pointed out by clang/Coverity

parse_context, parse_user_security: test for validity of results from
parse_ascii(..) with by casting to int32_t and comparing to -1; comparing
unsigned types to negative values will always be false.

Reported by: clang, Coverity
CID: 1011432, 1011433
MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division