BUG/MINOR: rules: Increment be_counters if backend is assigned for a silent-drop

Backend counters must be incremented only if a backend was already assigned to
the stream (when the stream exists). Otherwise, it means we are still on the
frontend side.

This patch may be backported as far as 1.6.
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 74790e8..afcb1bc 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -1266,6 +1266,8 @@
 		channel_abort(&strm->res);
 		strm->req.analysers &= AN_REQ_FLT_END;
 		strm->res.analysers &= AN_RES_FLT_END;
+		if (strm->flags & SF_BE_ASSIGNED)
+			_HA_ATOMIC_ADD(&strm->be->be_counters.denied_req, 1);
 		if (!(strm->flags & SF_ERR_MASK))
 			strm->flags |= SF_ERR_PRXCOND;
 		if (!(strm->flags & SF_FINST_MASK))