fix: dev: Improve the input validation of the isc_url_parse() function

The isc_url_parse() function failed to check the input buffer's
length and assumed that it can't be bigger than UINT16_MAX, because
both the 'off' and 'len' fields of the 'isc_url_parser_t' structure
are uint16_t.

Add a check to not accept a buffer longer than 8192 octets.

Closes #6150

Merge branch '6150-isc_url_parse-buffer-size-check-fix' into 'main'

See merge request isc-projects/bind9!12252