fix isCanonical on INLINE_COMPOSITE lists with zero-sized structs

c++/src/capnp/canonicalize-test.c++

@@ -185,6 +185,23 @@ KJ_TEST("isCanonical rejects unused trailing words") {
   KJ_ASSERT(!message.isCanonical());
 }
 
+KJ_TEST("isCanonical accepts empty inline composite list of zero-sized structs") {
+   AlignedData<3> segment = {{
+    // Struct pointer, pointer in next word
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
+
+    // List pointer, inline composite, zero words long
+    0x01, 0x00, 0x00, 0x00, 0x07, 0x00, 0x00, 0x00,
+
+    // Tag word
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  }};
+  kj::ArrayPtr<const word> segments[1] = {kj::arrayPtr(segment.words, 3)};
+  SegmentArrayMessageReader message(kj::arrayPtr(segments, 1));
+
+  KJ_ASSERT(message.isCanonical());
+}
+
 KJ_TEST("upgraded lists can be canonicalized") {
   AlignedData<7> upgradedList = {{
     //Struct pointer, data immediately follows, 4 pointer fields, no data

c++/src/capnp/layout.c++

@@ -2965,6 +2965,9 @@ bool ListReader::isCanonical(const word **readHead) {
       }
       auto structSize = (this->structDataSize / BITS_PER_WORD) +
                         (this->structPointerCount * WORDS_PER_POINTER);
+      if (structSize == 0 * WORDS) {
+        return true;
+      }
       auto listEnd = *readHead + (this->elementCount / ELEMENTS * structSize) / WORDS;
       auto pointerHead = listEnd;
       bool listDataTrunc = false;