Patch a bug in the subseq and revcomplement methods. That patchs the bug in the option -c of obipcr

Former-commit-id: 7999b917d07545271036af6e66f53aea27fc6e7b

pkg/obiseq/revcomp.go

@@ -43,6 +43,10 @@ func nucComplement(n byte) byte {
 // The function returns the reverse complemented BioSequence.
 func (sequence *BioSequence) ReverseComplement(inplace bool) *BioSequence {
 
+	if sequence == nil {
+		return nil
+	}
+
 	if !inplace {
 		sequence = sequence.Copy()
 	}