BUILD: debug: Avoid warnings in dev mode with -02 because of some BUG_ON tests

Some BUG_ON() tests emit a warning because of a potential null pointer
dereference on an HTX block. In fact, it should never happen, but now, GCC is
happy.

This patch must be backported to 2.0.

(cherry picked from commit ea009736d8f25be6c79aff0bed3a994eef87f3fd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/mux_h2.c b/src/mux_h2.c
index a0fb93d..fd039d1 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4586,7 +4586,7 @@
 
 	/* get the start line, we do have one */
 	blk = htx_get_head_blk(htx);
-	BUG_ON(htx_get_blk_type(blk) != HTX_BLK_RES_SL);
+	BUG_ON(!blk || htx_get_blk_type(blk) != HTX_BLK_RES_SL);
 	ALREADY_CHECKED(blk);
 	sl = htx_get_blk_ptr(htx, blk);
 	h2s->status = sl->info.res.status;
@@ -4795,7 +4795,7 @@
 
 	/* get the start line, we do have one */
 	blk = htx_get_head_blk(htx);
-	BUG_ON(htx_get_blk_type(blk) != HTX_BLK_REQ_SL);
+	BUG_ON(!blk || htx_get_blk_type(blk) != HTX_BLK_REQ_SL);
 	ALREADY_CHECKED(blk);
 	sl = htx_get_blk_ptr(htx, blk);
 	meth = htx_sl_req_meth(sl);