RFC 4271 4.3 says:

 The fourth high-order bit (bit 3) of the Attribute Flags octet
 is the Extended Length bit.  It defines whether the Attribute
 Length is one octet (if set to 0) or two octets (if set to 1).

 The lower-order four bits of the Attribute Flags octet are
 unused.  They MUST be zero when sent and MUST be ignored when
 received.

 If the Extended Length bit of the Attribute Flags octet is set
 to 0, the third octet of the Path Attribute contains the length
 of the attribute data in octets.

 If the Extended Length bit of the Attribute Flags octet is set
 to 1, the third and fourth octets of the path attribute contain
 the length of the attribute data in octets.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit 9397c36)