adversarial rx_parse tests; fix output-zeroing on empty payload

Add test_intrusive_rx.c with 16 golden adversarial tests for rx_parse,
covering all three protocol versions (v0.1, v1.0, v1.1), version
detection, tail-byte parsing, bit-field boundary checks, cross-version
ambiguity, and rejection paths.

Fix: move memset of output structs before the early-return size check
so that outputs are always zeroed regardless of payload size.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pavel-kirienko

libcanard/canard.c

@@ -1083,12 +1083,12 @@ static byte_t rx_parse(const uint32_t       can_id,
     CANARD_ASSERT(can_id < (UINT32_C(1) << 29U));
     CANARD_ASSERT(out_v0 != NULL);
     CANARD_ASSERT(out_v1 != NULL);
+    memset(out_v0, 0, sizeof(*out_v0));
+    memset(out_v1, 0, sizeof(*out_v1));
     if (payload.size < 1) {
         return 0;
     }
     CANARD_ASSERT(payload.data != NULL);
-    memset(out_v0, 0, sizeof(*out_v0));
-    memset(out_v1, 0, sizeof(*out_v1));
 
     // Common items.
     const canard_prio_t priority = (canard_prio_t)(can_id >> PRIO_SHIFT);

tests/CMakeLists.txt

@@ -99,6 +99,7 @@ add_compile_options(-Wno-unused-function)
 gen_test_matrix(test_helpers "src/test_helpers.c")
 gen_test_matrix(test_intrusive_util "src/test_intrusive_util.c")
 gen_test_matrix(test_intrusive_tx "src/test_intrusive_tx.c")
+gen_test_matrix(test_intrusive_rx "src/test_intrusive_rx.c")
 # API tests.
 gen_test_single(test_api_tx "${library_dir}/canard.c;src/test_api_tx.cpp")