Avoid nullptr subtraction using intptr_t casts

Andrew's review on the previous fix noted that the nullptr checks produced slightly worse codegen. Replace the pointer subtraction with intptr_t subtraction, which avoids the undefined behaviour of subtracting two null pointers without introducing extra branches.

src/parser3.h

@@ -83,26 +83,28 @@ struct Parser3 {
   [[nodiscard]] WeaselJsonStatus parse(char *buf, int len);
 
   void flushNumber(bool done, char *buf) {
-    int len = buf - dataBegin;
+    int len = (intptr_t)buf - (intptr_t)dataBegin;
     assert(len >= 0);
     if (done || len > 0) {
-      callbacks->on_number_data(userdata, dataBegin, len, done);
+      callbacks->on_number_data(userdata, dataBegin ? dataBegin : "", len,
+                                done);
     }
   }
 
   void flushString(bool done, char *buf) {
     int len;
     if (!(flags & WeaselJsonRaw)) {
-      len = writeBuf - dataBegin;
+      len = (intptr_t)writeBuf - (intptr_t)dataBegin;
     } else {
-      len = buf - dataBegin;
+      len = (intptr_t)buf - (intptr_t)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);