CTest: Fix our internal CURL_DEBUGFUNCTION to conform to CURL docs

The curl debug callback function must return ``0``.

Fixes: #20449

Source/CTest/cmCTestCurl.cxx

@@ -55,7 +55,7 @@ size_t curlDebugCallback(CURL* /*unused*/, curl_infotype /*unused*/,
                          char* chPtr, size_t size, void* data)
 {
   cm::append(*static_cast<std::vector<char>*>(data), chPtr, chPtr + size);
-  return size;
+  return 0;
 }
 }
 

Source/CTest/cmCTestSubmitHandler.cxx

@@ -110,7 +110,7 @@ static size_t cmCTestSubmitHandlerCurlDebugCallback(CURL* /*unused*/,
 {
   cm::append(*static_cast<cmCTestSubmitHandlerVectorOfChar*>(data), chPtr,
              chPtr + size);
-  return size;
+  return 0;
 }
 
 cmCTestSubmitHandler::cmCTestSubmitHandler()