With ext-ref enabled, it is possible that when trying to encode the
first true ALTREF frame after a keyframe, the previous ALTREF frame
(alias for the keyframe) is the same as one of the new LAST{2,3,4}
reference frames, and hence cpi->ref_frame_flags will have the ALTREF
bit clear, as computed by get_ref_frame_flags in encoder.c.

sf->alt_ref_search_fp forces the previous ALTREF frame to
be used as the only possible  reference when encoding a new ALTREF
frame, but due to cpi->ref_frame_flags, some buffers will not be
initialized (see rdopt.c:7689 yv12_mb), leading to a segfault.

get_ref_frame_flags in encoder.c has been changed to prefer to keep
the  LAST frame, then the ALTREF frame, then any of the LAST{2,3,4}
frames and then the GOLDEN frame in that order of preference in case
any of them are the same. This avoids the segfault and behaves the
same for the baseline.

Change-Id: I4da1991667614009da5d3061a6316c0d5dbc6c0c