lavf/vc1test: fix vc1test can't probe some RCV file.

lavf/vc1test: fix vc1test can't probe some RCV file.

case 1:
use the hexdump -C SMM0005.rcv get:
                     size              skip (size - 4)
                      |                        |
                      V                        V
00000000  18 00 00 c5 05 00 00 00  4d f1 0a 11 00 e0 01 00
00000010  00 d0 02 00 00 0c 00 00  00 88 13 00 00 c0 65 52
                         ^
			 |
		     size + 16
case 2:
same the command for SMM0015.rcv get:
                    size
                      |
                      V
00000000  19 00 00 c5 04 00 00 00  41 f3 80 01 40 02 00 00
00000010  d0 02 00 00 0c 00 00 00  00 00 00 10 00 00 00 00
                      ^
		      |
		   size + 16

There are different the RCV file format for VC-1, vc1test
just handle the case 2 now, this fix will support the case 1.
(Both of test clips come from: SMPTE Recommended Practice -
VC-1 Decoder and Bitstream Conformance). And I think I got
a older VC-1 test clip in the case 1.

Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Reviewed-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Yan, FengX <fengx.yan@intel.com>