Use mattermost v5 module (#1192)

vendor/github.com/go-asn1-ber/asn1-ber/header.go

@@ -0,0 +1,35 @@
+package ber
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+func readHeader(reader io.Reader) (identifier Identifier, length int, read int, err error) {
+	if i, c, err := readIdentifier(reader); err != nil {
+		return Identifier{}, 0, read, err
+	} else {
+		identifier = i
+		read += c
+	}
+
+	if l, c, err := readLength(reader); err != nil {
+		return Identifier{}, 0, read, err
+	} else {
+		length = l
+		read += c
+	}
+
+	// Validate length type with identifier (x.600, 8.1.3.2.a)
+	if length == LengthIndefinite && identifier.TagType == TypePrimitive {
+		return Identifier{}, 0, read, errors.New("indefinite length used with primitive type")
+	}
+
+	if length < LengthIndefinite {
+		err = fmt.Errorf("length cannot be less than %d", LengthIndefinite)
+		return
+	}
+
+	return identifier, length, read, nil
+}