OML: Work around OsmoBSC sending "GET ATTRIBUTES" with short length
OML: Work around OsmoBSC sending "GET ATTRIBUTES" with short length

OsmoBSC used to have a bug in encoding the "GET ATTRIBUTES" OML message,
resulting in the actual message length being three bytes longer than the
encoded length value.

As in Ib98f0d7c2cff9172714ed18667c02564540d65d7 we have introduced
proper consistency checks on length values, all "GET ATTRIBUTES" from
OsmoBSC version suntil today will fail.  This patch introduces a
work-around to remain compatible with old OsmoBSC while still keeping
the consistency checks for all other messages.

Change-Id: Ifa24e9e2c71feb2c597557807d675438c2825b2d
Related: OS#3799

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts