Out-of-memory in QXmlStreamReader

1. Build the attached project.
2. To visualize the issue, restrict the available memory:

   ulimit -Sv 3200000
   

   That's about 3GB.

3. Run the resulting program and pass one of the attached xml files:

   ./report 30924.xml
   

   You will see output like:

   terminate called after throwing an instance of 'std::bad_alloc'
     what():  std::bad_alloc
   Aborted (core dumped)
   

On Qt 5.15, the program still finishes successfully with

ulimit -Sv 2300000

which is 28% less memory.

Bisecting qtbase resulted in:

There are only 'skip'ped commits left to test.
The first bad commit could be any of:
3398eeadf617880af38fb540f26273df87ba0b1f
8dc7761e6d490877af18949c0177097e5c857424
ecfb5d2d15e586c40ca2e8b097fb77821be8c884

Google's oss-fuzz found these as issues 29302 and 30924.