With the description in frame size with refs semantics (SPEC 7.2.5),
it is a requirement of bitstream conformance that for at least one
reference frame has the valid dimensions.

Modify the check to make sure the decoder works well in SINGLE_REFERENCE
mode that not all reference frames have valid dimensions.

Check and error out if invalid reference frame is used in inter_recon.

One of the failure case is a 480x272 inter frame (SINGLE_REFERENCE mode)
with following reference pool:

0.  960x544    LAST    valid
1. 1920x1088 GOLDEN  invalid, but not used in single reference mode
2. 1920x1088 ALTREF  invalid, but not used in single reference mode
3~7  ...     Unused

Identical logic in libvpx:
<https://github.com/webmproject/libvpx/blob/master/vp9/decoder/vp9_decodeframe.c#L736>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>