Prefix-integer encoding function was incomplete. It was not able to deal
correctly with value encoded on more than 2 bytes. This maximum value depends
on the size of the prefix, but value greater than 254 were all impacted.
Most notably, this change is required to support header name/value with
sizeable length. Previously, length was incorrectly encoded. The client thus
closed the connection with QPACK_DECOMPRESSION_ERROR.
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.
