[9.20] fix: dev: Fix isc_buffer_init capacity mismatch in DoH data chunk callback

[9.20] fix: dev: Fix isc_buffer_init capacity mismatch in DoH data chunk callback

isc_buffer_init() is given MAX_DNS_MESSAGE_SIZE (65535) as capacity but
only h2->content_length bytes are allocated.  This makes the buffer
believe it has more space than actually allocated.  A secondary bounds
check (new_bufsize <= h2->content_length) prevents actual overflow, but
the buffer invariant is violated.

Pass h2->content_length as the capacity to match the allocation.

Backport of MR !11662

Merge branch 'backport-ondrej/fix-isc_buffer_init-capacity-mismatch-in-DoH-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11709