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
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  avio: Use AVERROR_PROTOCOL_NOT_FOUND

Conflicts:
	libavformat/avio.c

See: 12eeced8b41bbe46693f8c9625ed61b51f8438a1
Merged-by: Michael Niedermayer <michaelni@gmx.at>