BUG/MAJOR: fcgi: Fix uninitialized reserved bytes

The output buffer is not zero-initialized. If we don't clear reserved
bytes, fcgi requests sent to backend will leak sensitive data.

This patch must be backported as far as 2.2.
diff --git a/src/fcgi.c b/src/fcgi.c
index dcf2db2..1d1a82b 100644
--- a/src/fcgi.c
+++ b/src/fcgi.c
@@ -47,7 +47,7 @@
 	out->area[len++] = ((h->len >> 8) & 0xff);
 	out->area[len++] = (h->len & 0xff);
 	out->area[len++] = h->padding;
-	len++; /* rsv */
+	out->area[len++] = 0; /* rsv */
 
 	out->data = len;
 	return 1;
@@ -94,7 +94,11 @@
 	out->area[len++] = ((r->role >> 8) & 0xff);
 	out->area[len++] = (r->role & 0xff);
 	out->area[len++] = r->flags;
-	len += 5; /* rsv */
+	out->area[len++] = 0; /* rsv */
+	out->area[len++] = 0;
+	out->area[len++] = 0;
+	out->area[len++] = 0;
+	out->area[len++] = 0;
 
 	out->data = len;
 	return 1;