MINOR: htx: add an unchecked version of htx_get_head_blk()

MINOR: htx: add an unchecked version of htx_get_head_blk()

htx_get_head_blk() is used at plenty of places, many of which are known
to be safe, but the function checks for the presence of a first block
and returns NULL if it doesn't exist. While it's properly used, it makes
compilers complain at -Os on stream.c and mux_fcgi.c because they probably
don't propagate variables far enough to see that there's no risk.

Let's add an unchecked version for these use cases.