Clear random buf in olm_init_outbound_group_session

All the other methods clear their random inputs. This one needs to do the same,
to reduce the risk of the randomness being used elsewhere and leaking key info.

include/olm/outbound_group_session.h

@@ -96,7 +96,7 @@ size_t olm_init_outbound_group_session_random_length(
  */
 size_t olm_init_outbound_group_session(
     OlmOutboundGroupSession *session,
-    uint8_t const * random, size_t random_length
+    uint8_t *random, size_t random_length
 );
 
 /**

src/outbound_group_session.c

@@ -154,20 +154,23 @@ size_t olm_init_outbound_group_session_random_length(
 
 size_t olm_init_outbound_group_session(
     OlmOutboundGroupSession *session,
-    uint8_t const * random, size_t random_length
+    uint8_t *random, size_t random_length
 ) {
+    const uint8_t *random_ptr = random;
+
     if (random_length < olm_init_outbound_group_session_random_length(session)) {
         /* Insufficient random data for new session */
         session->last_error = OLM_NOT_ENOUGH_RANDOM;
         return (size_t)-1;
     }
 
-    megolm_init(&(session->ratchet), random, 0);
-    random += MEGOLM_RATCHET_LENGTH;
+    megolm_init(&(session->ratchet), random_ptr, 0);
+    random_ptr += MEGOLM_RATCHET_LENGTH;
 
-    _olm_crypto_ed25519_generate_key(random, &(session->signing_key));
-    random += ED25519_RANDOM_LENGTH;
+    _olm_crypto_ed25519_generate_key(random_ptr, &(session->signing_key));
+    random_ptr += ED25519_RANDOM_LENGTH;
 
+    _olm_unset(random, random_length);
     return 0;
 }