Return an error if Fileno is called with NULL instead of segfaulting.

--- ./rpmio/rpmio.c.orig	2005-01-26 03:39:58.000000000 +0000
+++ ./rpmio/rpmio.c	2005-12-16 17:51:19.000000000 +0000
@@ -3094,6 +3164,7 @@ int Fileno(FD_t fd)
 {
     int i, rc = -1;
 
+    if (fd == NULL) return -1;
     if (fd->req != NULL)
 	rc = 123456789;	/* HACK: https has no steenkin fileno. */
     else