Add more error checks on taxonomy reading

Former-commit-id: 30c848d8e61214afcc7d92f0d3a3f5a1474fc720

pkg/obitax/lca.go

@@ -11,6 +11,14 @@ import (
 )
 
 func (t1 *TaxNode) LCA(t2 *TaxNode) (*TaxNode, error) {
+	if t1 == nil {
+		log.Fatalf("Try to get LCA of nil taxon")
+	}
+
+	if t2 == nil {
+		log.Fatalf("Try to get LCA of nil taxon")
+	}
+
 	p1, err1 := t1.Path()
 
 	if err1 != nil {