Fix isc_buffer_init capacity mismatch in DoH data chunk callback

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.

(cherry picked from commit 8e240bbb5ff563b1caaa13afdd1338079e2d751b)