dtls.c: initialize hmac storage in dtls_init()

Raul Fuentes has pointed out[1] that dtls_hmac_storage_init() must
be called when working with static memory allocation. The proposed
fix was to call this function from dtls_init().

[1] http://dev.eclipse.org/mhonarc/lists/tinydtls-dev/msg00159.html

Change-Id: I6916905d0e968089a531b09c2c08b97b7a9c8a68