Segfault with derep_id on both strands

[torognes]

Frédéric discovered that vsearch 2.28.1 crashes with a segmentation fault when running derep_id on both strands.

It can be reproduced like this:

printf ">s1;size=1;\nA\n>s1;size=1;\nT\n" | \
   vsearch \
       --derep_id - \
       --minseqlength 1 \
       --strand both \
       --quiet \
       --output /dev/null