Check length of input in WithName

There is a constraint on the length of the name in a reference, so
WithName should be checking this instead of potentially creating a
reference that doesn't comply with the rules.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

reference/reference.go

@@ -204,6 +204,9 @@ func ParseNamed(s string) (Named, error) {
 // WithName returns a named object representing the given string. If the input
 // is invalid ErrReferenceInvalidFormat will be returned.
 func WithName(name string) (Named, error) {
+	if len(name) > NameTotalLengthMax {
+		return nil, ErrNameTooLong
+	}
 	if !anchoredNameRegexp.MatchString(name) {
 		return nil, ErrReferenceInvalidFormat
 	}