BUILD: htx: use the unchecked version of htx_get_head_blk() where needed

stream.c and mux_fcgi.c may cause a warning for a possible NULL deref
at -Os, while that is not possible thanks to the previous test. Let's
just switch to __htx_get_head_blk() instead.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 26240cf..bb9ec56 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -462,7 +462,7 @@
 
 	/* Display status-line if possible (verbosity > MINIMAL) */
 	if (src->verbosity > FCGI_VERB_MINIMAL && htx && htx_nbblks(htx)) {
-		const struct htx_blk *blk = htx_get_head_blk(htx);
+		const struct htx_blk *blk = __htx_get_head_blk(htx);
 		const struct htx_sl  *sl  = htx_get_blk_ptr(htx, blk);
 		enum htx_blk_type    type = htx_get_blk_type(blk);
 
diff --git a/src/stream.c b/src/stream.c
index e20ac98..f3f776d 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -187,7 +187,7 @@
 
 	/* If msg defined, display status-line if possible (verbosity > MINIMAL) */
 	if (src->verbosity > STRM_VERB_MINIMAL && htx && htx_nbblks(htx)) {
-		const struct htx_blk *blk = htx_get_head_blk(htx);
+		const struct htx_blk *blk = __htx_get_head_blk(htx);
 		const struct htx_sl  *sl  = htx_get_blk_ptr(htx, blk);
 		enum htx_blk_type    type = htx_get_blk_type(blk);