speedhq: fix out-of-bounds write

Certain alpha run lengths (for SHQ1/SHQ3/SHQ5) could be stored in
both long and short versions, and we would only accept the short version,
returning -1 (invalid code) for the others. This could cause an
out-of-bounds write on malicious input, as discovered by
Andreas Cadhalpun during fuzzing.

Fix by simply allowing both versions, leaving no invalid codes
in the alpha VLC.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
