Fix check for term having _prefix.

Author: gkellogg

lib/compact.js

@@ -827,7 +827,7 @@ api.compactIri = ({
   // If iri could be confused with a compact IRI using a term in this context,
   // signal an error
   for(const [term, td] of activeCtx.mappings) {
-    if(td._prefix && iri.startsWith(term + ':')) {
+    if(td && td._prefix && iri.startsWith(term + ':')) {
       throw new JsonLdError(
         (`Absolute IRI "${iri}" confused with prefix "${term}".`),
         'jsonld.SyntaxError',