hmac: Reset the underlying hasher before doing set_key() with longer keys

The user might have done a non-complete append, having some state in the
hasher.

Fixes #909.

src/libstrongswan/plugins/hmac/hmac.c

@@ -103,7 +103,8 @@ METHOD(mac_t, set_key, bool,
 	if (key.len > this->b)
 	{
 		/* if key is too long, it will be hashed */
-		if (!this->h->get_hash(this->h, key, buffer))
+		if (!this->h->reset(this->h) ||
+			!this->h->get_hash(this->h, key, buffer))
 		{
 			return FALSE;
 		}