Bugfix: result of 0 4mers not caught if sequence length == 3

In the 4mer calculation:
length := slength - 3

- for sequences with <4 bases, length is <=0

The check to stop did only catch <0, so sequences lengths 2 or less, leaving sequence lengths of 3 unguarded
if length < 0 {
		return nil
	}

pkg/obikmer/encodefourmer.go

@@ -47,7 +47,7 @@ func Encode4mer(seq *obiseq.BioSequence, buffer *[]byte) []byte {
 	length := slength - 3
 	rawseq := seq.Sequence()
 
-	if length < 0 {
+	if length <= 0 {
 		return nil
 	}