avcodec/magicyuv: Don't invert order unnecessarily

avcodec/magicyuv: Don't invert order unnecessarily

The MagicYUV decoder currently sets both the length and the symbol field
of an array of HuffEntries; hereby the symbol of the ith entry (0-based)
is just i. Then said array gets sorted so that entries with greater
length are at the end and entries with the same length are ordered so
that those with smaller symbols are at the end. Afterwards the newly
sorted array is traversed in reverse order. This commit instead inverts
the ordering and traverses the array in its ordinary order in order to
simplify understanding.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>