X509_check_mumble() failure is <= 0, not just 0.

(This change is for a future change that increases the range of the
return values.)

(Imported from upstream's 3fc0b1edad0c75d7beb51fa77f63ffe817295e2c.)

Change-Id: I221d4ee0e90586f89f731e01ff4d813058173211