dtls: contiki dtls_retransmit_process: remove node before transmit

Mimic the default 'dtls_check_retransmit' implementation.
The node should be popped before calling 'retransmit' and not freed after
transmit.

Change-Id: If1c11184e27025dc69c672588b946e3cf61054e1
Signed-off-by: Gaëtan Harter <gaetan.harter@inria.fr>

dtls.c

diff --git a/dtls.c b/dtls.c
index 2da67f8..896ee90 100644
--- a/dtls.c
+++ b/dtls.c
@@ -4097,9 +4097,8 @@
 	
 	now = clock_time();
 	if (node && node->t <= now) {
+	  netq_pop_first(&the_dtls_context.sendqueue);
 	  dtls_retransmit(&the_dtls_context, node);
-
-          netq_node_free(node);
 	  node = netq_head(&the_dtls_context.sendqueue);
 	}