QCborValue::fromCbor tries to allocate 15 TB of memory

1. Have a build of Qt with configure flags "-sanitize address"
2. Build the attached project on this build of Qt.
3. Set any ulimit for the size of virtual memory, e.g.:

   ulimit -Sv 200000000

   Yes, that's two hundred gigabytes.

4. Run the resulting program and pass the input file:

   ./cbor input.cbor

   You'll get output like the following:

   ==9238==ERROR: AddressSanitizer failed to allocate 0xdfff0001000
   (15392894357504) bytes at address 2008fff7000 (errno: 12)
   ==9238==ReserveShadowMemoryRange failed while trying to map
   0xdfff0001000 bytes. Perhaps you're using ulimit -v
   Aborted (core dumped)
   

Even if this only seems to happen when built with ASAN and having set an
ulimit, trying to allocate 15 TB seems pretty wrong to me.

Google's oss-fuzz found this as issue 21067.