fixup! check n_sigs in partial_sig_agg

Author: jonasnick

src/modules/musig/session_impl.h

@@ -735,6 +735,7 @@ int secp256k1_musig_partial_sig_agg(const secp256k1_context* ctx, unsigned char
     ARG_CHECK(sig64 != NULL);
     ARG_CHECK(session != NULL);
     ARG_CHECK(partial_sigs != NULL);
+    ARG_CHECK(n_sigs > 0);
 
     if (!secp256k1_musig_session_load(ctx, &session_i, session)) {
         return 0;

src/modules/musig/tests_impl.h

@@ -524,7 +524,8 @@ void musig_api_tests(secp256k1_scratch_space *scratch) {
     CHECK(ecount == 4);
     CHECK(secp256k1_musig_partial_sig_agg(none, pre_sig, &session, invalid_partial_sig_ptr, 2) == 0);
     CHECK(ecount == 5);
-    CHECK(secp256k1_musig_partial_sig_agg(none, pre_sig, &session, partial_sig_ptr, 0) == 1);
+    CHECK(secp256k1_musig_partial_sig_agg(none, pre_sig, &session, partial_sig_ptr, 0) == 0);
+    CHECK(ecount == 6);
     CHECK(secp256k1_musig_partial_sig_agg(none, pre_sig, &session, partial_sig_ptr, 1) == 1);
     CHECK(secp256k1_musig_partial_sig_agg(none, pre_sig, &session, partial_sig_ptr, 2) == 1);