enforce symmetry in unitigs

[boisvert@bigmem biosal]$ ./scripts/assembly_debugging/check-names.sh 1.fasta
Got names
Got defect names
Check DefectList.txt (defects: 28 / 5354)
4 longest:
9362
8170
6652
6652

Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>