BUG/MINOR: http_htx: Remove BUG_ON() from http_get_stline() function

The http_get_stline() was designed to be called from HTTP analyzers. Thus
before any data forwarding. To prevent any invalid usage, two BUG_ON()
statements were added. However, it is not a good idea because it is pretty
hard to be sure no HTTP sample fetch will never be called outside the
analyzers context. Especially because there is at least one possible area
where it may happens. An HTTP sample fetch may be used inside the unique-id
format string. On the normal case, it is generated in AN_REQ_HTTP_INNER
analyzer. But if an error is reported too early, the id is generated when
the log is emitted.

So, it is safer to remove the BUG_ON() statements and consider the normal
behavior is to return NULL if the first block is not a start-line. Of
course, this means all calling functions must test the return value or be
sure the start-line is really there.

This patch must be backported as far as 2.0.
diff --git a/src/http_htx.c b/src/http_htx.c
index 53732d1..2ddc50e 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -64,11 +64,9 @@
 {
 	struct htx_blk *blk;
 
-	BUG_ON(htx->first == -1);
 	blk = htx_get_first_blk(htx);
-	if (!blk)
+	if (!blk || (htx_get_blk_type(blk) != HTX_BLK_REQ_SL && htx_get_blk_type(blk) != HTX_BLK_RES_SL))
 		return NULL;
-	BUG_ON(htx_get_blk_type(blk) != HTX_BLK_REQ_SL && htx_get_blk_type(blk) != HTX_BLK_RES_SL);
 	return htx_get_blk_ptr(htx, blk);
 }