Fix for #17277 (#21819)

Fixed adding a string with a large number of trailing zeros to StringBuilder, which sometimes caused the thread to hang

Author: artar94

src/Middleware/SpaServices.Extensions/src/Util/EventedStreamReader.cs

@@ -111,7 +111,7 @@ private async Task Run()
                 // get the rest
                 if (lineBreakPos < 0 && startPos < chunkLength)
                 {
-                    _linesBuffer.Append(buf, startPos, chunkLength);
+                    _linesBuffer.Append(buf, startPos, chunkLength - startPos);
                 }
             }
         }