MINOR: applet: Add appctx_init() helper fnuction

It is just a helper function that call the .init applet callback function,
if it exists. This will simplify a bit the init stage when a new applet is
started. For now, this callback function is only used when a new service is
started.
diff --git a/include/haproxy/applet.h b/include/haproxy/applet.h
index 2ee368b..bef5aa5 100644
--- a/include/haproxy/applet.h
+++ b/include/haproxy/applet.h
@@ -42,6 +42,16 @@
 
 struct appctx *appctx_new(struct applet *applet, struct cs_endpoint *endp);
 
+/* Helper function to call .init applet callback function, if it exists. Returns 0
+ * on success and -1 on error.
+ */
+static inline int appctx_init(struct appctx *appctx)
+{
+	if (appctx->applet->init)
+		return appctx->applet->init(appctx);
+	return 0;
+}
+
 /* Releases an appctx previously allocated by appctx_new(). */
 static inline void __appctx_free(struct appctx *appctx)
 {
diff --git a/src/stream.c b/src/stream.c
index dd3358e..29d1a60 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -997,9 +997,8 @@
 			return ACT_RET_ERR;
 
 		/* Finish initialisation of the context. */
-		memset(&appctx->ctx, 0, sizeof(appctx->ctx));
 		appctx->rule = rule;
-		if (appctx->applet->init && !appctx->applet->init(appctx))
+		if (appctx_init(appctx) == -1)
 			return ACT_RET_ERR;
 	}
 	else