MINOR: applet: Use unsafe version to get stream from SC in the trace function

When a trace message for an applet is dumped, if the SC exists, the stream
always exists too. There is no way to attached an applet to a health-check.
So, we can use the unsafe version __sc_strm() to get the stream.

This patch is related to #2106. Not sure it will be enough for
Coverity. However, there is no bug here.
diff --git a/src/applet.c b/src/applet.c
index 41f1b08..a4702d4 100644
--- a/src/applet.c
+++ b/src/applet.c
@@ -105,7 +105,7 @@
 
 	sc = appctx_sc(appctx);
 	if (sc) {
-		s = sc_strm(sc);
+		s = __sc_strm(sc);
 		sco = sc_opposite(sc);
 		ic = sc_ic(sc);
 		oc = sc_oc(sc);