ssh: Add explicit type in comparison with constant to make go-fuzz happy

Currently using go-fuzz with code using golang.org/x/crypto/ssh fails
because it passes CertTimeInfinity to an interface{} and automatically
tries to use an int, which overflows and results in a compile error.

This change adds a no-op type conversion inside the function which
makes things compile with go-fuzz.

Change-Id: Iade0c0df7e20cbac4928480fad3e44c831e1e00a
Reviewed-on: https://go-review.googlesource.com/11285
Reviewed-by: Adam Langley <agl@golang.org>

ssh/certs.go

@@ -368,7 +368,7 @@ func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
 	if after := int64(cert.ValidAfter); after < 0 || unixNow < int64(cert.ValidAfter) {
 		return fmt.Errorf("ssh: cert is not yet valid")
 	}
-	if before := int64(cert.ValidBefore); cert.ValidBefore != CertTimeInfinity && (unixNow >= before || before < 0) {
+	if before := int64(cert.ValidBefore); cert.ValidBefore != uint64(CertTimeInfinity) && (unixNow >= before || before < 0) {
 		return fmt.Errorf("ssh: cert has expired")
 	}
 	if err := cert.SignatureKey.Verify(cert.bytesForSigning(), cert.Signature); err != nil {