Add an explicit initializer for the musigsession signing state.

This is redundant with the current implementation. However, the
musigsession module is written in such a way that the calling code
has no knowledge about its internal working. Therefore, it should
not assume that zeroing out is the correct way of initializing it.

src/handler/sign_psbt.c

@@ -2906,6 +2906,9 @@ void handler_sign_psbt(dispatcher_context_t *dc, uint8_t protocol_version) {
     signing_state_t signing_state;
     memset(&signing_state, 0, sizeof(signing_state));
 
+    // Make sure that the signing state for MuSig2 is initialized correctly
+    musigsession_initialize_signing_state(&signing_state.musig);
+
     // compute all the tx-wide hashes
     if (!compute_tx_hashes(dc, &st, &signing_state.tx_hashes)) {
         return;

src/musig/musig_sessions.c

@@ -83,6 +83,10 @@ void compute_rand_i_j(const musig_psbt_session_t *psbt_session,
     crypto_hash_digest(&hash_context.header, out, 32);
 }
 
+void musigsession_initialize_signing_state(musig_signing_state_t *musig_signing_state) {
+    memset(musig_signing_state, 0, sizeof(musig_signing_state_t));
+}
+
 const musig_psbt_session_t *musigsession_round1_initialize(
     uint8_t psbt_session_id[static 32],
     musig_signing_state_t *musig_signing_state) {
@@ -126,6 +130,9 @@ void musigsession_commit(musig_signing_state_t *musig_signing_state) {
     for (size_t i = 0; i < sizeof(musig_signing_state->_round1); i++) {
         acc |= musig_signing_state->_round1._id[i];
     }
+    // If round 1 was not executed, then there is nothing to store.
+    // This assumes that musigsession_initialize_signing_state, therefore the field is zeroed out
+    // if it wasn't used.
     if (acc != 0) {
         musigsession_store(musig_signing_state->_round1._id, &musig_signing_state->_round1);
     }

src/musig/musig_sessions.h

@@ -38,6 +38,20 @@ void compute_rand_i_j(const musig_psbt_session_t *psbt_session,
                       int placeholder_index,
                       uint8_t out[static 32]);
 
+/**
+ * Make sure that the musig signing state is initialized correctly.
+ *
+ * This method must be called before musigsession_round1_initialize or
+ * musigsession_round2_initialize are called in the code.
+ *
+ * This allows the calling code to not make any assumption about how
+ * the inialization of the musig signing state is done.
+ *
+ * @param[in]  musig_signing_state
+ *   Pointer to the musig signing state.
+ */
+void musigsession_initialize_signing_state(musig_signing_state_t *musig_signing_state);
+
 /**
  * Handles the creation of a new musig psbt session into the volatile memory, or its retrieval (if
  * the session already exists).
@@ -80,8 +94,6 @@ __attribute__((warn_unused_result)) const musig_psbt_session_t *musigsession_rou
  * been returned to the client. It must _not_ be called if any error occurs, or if the signing
  * process is aborted for any reason.
  *
- * @param[in]  psbt_session_id
- *   Pointer to the musig psbt session id.
  * @param[in]  musig_signing_state
  *   Pointer to the musig signing state.
  */