Don't parse XML symbols longer than 4096 characters

It is slow and will use too much memory.

Pick-to: 6.1 6.1.0 6.0 5.15
Fixes: QTBUG-91889
Change-Id: I45c5e6038357c87bbb85b1ace17ef39a2a814ea0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qxmlstream.cpp

@@ -1297,6 +1297,11 @@ inline int QXmlStreamReaderPrivate::fastScanName(int *prefix)
     int n = 0;
     uint c;
     while ((c = getChar()) != StreamEOF) {
+        if (n >= 4096) {
+            // This is too long to be a sensible name, and
+            // can exhaust memory
+            return 0;
+        }
         switch (c) {
         case '\n':
         case ' ':