check_snmp: Fix support for inverse thresholds

Commit bd782990566eec91b8312cfc2765a7e2bd9e67da reintroduced support for
inverse threshold ranges such as "2:1", but it broke standard thresholds
such as "1:2" (by converting this range into "1") or "1:" (by converting
this range into "@:2").  This commit fixes those two bugs, plus an
off-by-one error while computing the number of bytes to allocate when
handling inverse thresholds (two additional bytes were allocated where
three were required to hold '@' and ':' and '\0').  While at it, we also
check whether malloc(3) succeeded.