Validate chunk size and reject malformed or out-of-range values

getChunkSize() parsed the chunk-size line with toLong(&ok, 16) and
ignored ok.

Parse with toULongLong() at qsizetype width, check ok, and reject a
value that does not fit qsizetype with 400 Bad Request.

Task-number: QTBUG-150270
Pick-to: 6.12 6.12.0 6.11
Change-Id: I2b42a7971f2bb2f909477bb7e1a6cb120be2aca5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>