dtls.c: fix memory leak in dtls_free_context()

In [1], Anton Meier points out that the nodes in the sendqueue are
not freed when the context is destroyed. This change adds the
suggested fix.

[1] https://dev.eclipse.org/mhonarc/lists/tinydtls-dev/msg00139.html

Change-Id: Ia248b0fbe4c5adb56ee883ac05001eefc67c0d8c