BUG/MINOR: qpack: fix buffer API usage on prefix integer encoding

BUG/MINOR: qpack: fix buffer API usage on prefix integer encoding

Replace bogus call b_data() by b_room() to check if there is enough
space left in the buffer before encoding a prefix integer.

At this moment, no real scenario was found to trigger a bug related to
this change. This is probably because the buffer always contains data
(field section line and status code) before calling
qpack_encode_prefix_integer() which prevents an occurrence of this bug.