BUG/MINOR: httpclient/lua: error when the httpclient_start() fails

Jump to an luaL_error() when the httpclient fails, it could be the
result of an allocation failure, or even a wrong URL.

Must be backported in 2.5.
diff --git a/src/hlua.c b/src/hlua.c
index a2f5ca7..3dd2667 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -7300,7 +7300,8 @@
 		return 0;
 	}
 
-	httpclient_start(hlua_hc->hc);
+	if (!httpclient_start(hlua_hc->hc))
+		WILL_LJMP(luaL_error(L, "couldn't start the httpclient"));
 
 	MAY_LJMP(hlua_yieldk(L, 0, 0, hlua_httpclient_snd_yield, TICK_ETERNITY, 0));