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.

include/haproxy/htx.h

@@ -189,6 +189,16 @@ static inline struct htx_blk *htx_get_head_blk(const struct htx *htx)
 	return ((head == -1) ? NULL : htx_get_blk(htx, head));
 }
 
+/* same as above but unchecked, may only be used when certain that a block
+ * exists.
+ */
+static inline struct htx_blk *__htx_get_head_blk(const struct htx *htx)
+{
+	int32_t head = htx_get_head(htx);
+
+	return htx_get_blk(htx, head);
+}
+
 /* Returns the type of the oldest HTX block (head) if the HTX message is not
  * empty. Otherwise it returns HTX_BLK_UNUSED.
  */