BUG/MINOR: proxy: unsafe initialization of HTTP transaction when switching from TCP frontend

A switch from a TCP frontend to an HTTP backend initializes the HTTP
transaction. txn->hdr_idx.size is used by hdr_idx_init() but not
necessarily initialized yet here, because the first call to hdr_idx_init()
is in fact placed in http_init_txn(). Moving it before the call is
enough to fix it. We also remove the useless extra confusing call
to hdr_idx_init().

The bug was introduced in 1.5-dev8 with commit ac1932d ("MEDIUM:
tune.http.maxhdr makes it possible to configure the maximum number
of HTTP headers"). No backport to stable is needed.
diff --git a/src/proxy.c b/src/proxy.c
index fb1a3b4..c8b815e 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -925,14 +925,12 @@
 	 * a struct hdr_idx for it if we did not have one.
 	 */
 	if (unlikely(!s->txn.hdr_idx.v && be->http_needed)) {
+		s->txn.hdr_idx.size = global.tune.max_http_hdr;
 		if ((s->txn.hdr_idx.v = pool_alloc2(pool2_hdr_idx)) == NULL)
 			return 0; /* not enough memory */
 
 		/* and now initialize the HTTP transaction state */
 		http_init_txn(s);
-
-		s->txn.hdr_idx.size = global.tune.max_http_hdr;
-		hdr_idx_init(&s->txn.hdr_idx);
 	}
 
 	/* If an LB algorithm needs to access some pre-parsed body contents,