avio: Use AVERROR_PROTOCOL_NOT_FOUND

When the protocol is missing ffurl_alloc() should return
AVERROR_PROTOCOL_NOT_FOUND instead of AVERROR(ENOENT).

Bug-Id: 577
CC: libav-stable@libav.org

libavformat/avio.c

@@ -197,7 +197,7 @@ int ffurl_alloc(URLContext **puc, const char *filename, int flags,
             return url_alloc_for_protocol (puc, up, filename, flags, int_cb);
     }
     *puc = NULL;
-    return AVERROR(ENOENT);
+    return AVERROR_PROTOCOL_NOT_FOUND;
 }
 
 int ffurl_open(URLContext **puc, const char *filename, int flags,