security/vuxml: Document QtNetwork buffer overflow

An issue was discovered in the HTTP2 implementation in Qt before 5.15.17,
6.x before 6.2.11, 6.3.x through 6.5.x before 6.5.4, and 6.6.x before
6.6.2. network/access/http2/hpacktable.cpp has an incorrect HPack integer
overflow check.

Base Score: 9.8 CRITICAL
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H