Avoid nullptr subtraction when flushing scalars at EOF

When a scalar ends exactly at a chunk boundary, Parser3::parse resets
dataBegin and writeBuf to the new buf at the start of every call. On the
EOF call buf is null, so both pointers become null. The final
flushNumber/flushString then computed len as buf - dataBegin, i.e.
nullptr - nullptr, which is undefined behaviour in C++.

Compute the flush length safely: if dataBegin is null (or, for raw mode,
buf is null), treat the length as zero. Use an empty string literal as a
non-null data pointer for the zero-length, done=true callback so callers
still receive the completion signal.

Add a regression test covering a number that fills its chunk exactly and
is finalized by an EOF call.

Fixes #40

src/parser3.h

@@ -83,26 +83,33 @@ struct Parser3 {
   [[nodiscard]] WeaselJsonStatus parse(char *buf, int len);
 
   void flushNumber(bool done, char *buf) {
-    int len = buf - dataBegin;
-    assert(len >= 0);
-    if (done || len > 0) {
-      callbacks->on_number_data(userdata, dataBegin, len, done);
-    }
-  }
-
-  void flushString(bool done, char *buf) {
-    int len;
-    if (!(flags & WeaselJsonRaw)) {
-      len = writeBuf - dataBegin;
-    } else {
+    int len = 0;
+    if (dataBegin != nullptr && buf != nullptr) {
       len = buf - dataBegin;
     }
     assert(len >= 0);
     if (done || len > 0) {
+      callbacks->on_number_data(userdata, dataBegin ? dataBegin : "", len,
+                                done);
+    }
+  }
+
+  void flushString(bool done, char *buf) {
+    int len = 0;
+    if (dataBegin != nullptr) {
+      if (!(flags & WeaselJsonRaw)) {
+        len = writeBuf - dataBegin;
+      } else if (buf != nullptr) {
+        len = buf - dataBegin;
+      }
+    }
+    assert(len >= 0);
+    if (done || len > 0) {
+      const char *data = dataBegin ? dataBegin : "";
       if (inKey) {
-        callbacks->on_key_data(userdata, dataBegin, len, done);
+        callbacks->on_key_data(userdata, data, len, done);
       } else {
-        callbacks->on_string_data(userdata, dataBegin, len, done);
+        callbacks->on_string_data(userdata, data, len, done);
       }
     }
   }

src/test.cpp

@@ -303,6 +303,24 @@ TEST_CASE("reset clears inKey and transient state") {
   WeaselJsonParser_destroy(parser);
 }
 
+TEST_CASE("scalar ending at chunk boundary is finalized at EOF") {
+  // A number whose digits exactly fill the first chunk must not invoke
+  // undefined behaviour on the EOF call, and must still signal completion.
+  auto c = serializeCallbacks();
+  SerializeState state;
+  auto *parser = WeaselJsonParser_create(1024, &c, &state, 0);
+  REQUIRE(parser != nullptr);
+
+  std::string chunk = "123";
+  REQUIRE(WeaselJsonParser_parse(parser, chunk.data(), chunk.size()) ==
+          WeaselJson_AGAIN);
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) == WeaselJson_OK);
+
+  CHECK(state.result == "(123)");
+
+  WeaselJsonParser_destroy(parser);
+}
+
 void doTestUnescapingUtf8(std::string const &escaped,
                           std::string const &expected, int stride, int flags) {
   CAPTURE(escaped);