contrib: bump opendht

* fix potential invalid iterator access
* reduce mtu to 1280 bytes

Change-Id: I23ef1ef5be439068a03f25985724431efc82f66e
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

contrib/src/opendht/rules.mak

 # OPENDHT
-OPENDHT_VERSION := 2ed99db9b1ec167327ef8fd7bfa2603b4f1ea009
+OPENDHT_VERSION := 2ca73023528a62bf789b5e7baa1f886109af15f2
 OPENDHT_URL := https://github.com/savoirfairelinux/opendht/archive/$(OPENDHT_VERSION).tar.gz
 
 PKGS += opendht