QWebSocket: fix off-by-one header size limit

The usual test would be something like `size < limit: OK`.
So the inverse is of course `size >= limit: NOK`.

[ChangeLog][QWebSocketServer/QWebSocket] A handshake header whose size is
exactly the limit (100 lines of 8 KiB plus the terminator) is now refused
rather than accepted.

Pick-to: 6.12 6.11 6.8 6.5
Task-number: QTBUG-150324
Change-Id: I3a99e2dec0bfac13227f4086915144feb84c6adc
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>