fix null pointer reference

git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@2247 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

C/iopreds.c

@@ -3870,7 +3870,8 @@ static Int
       } else {
 	if (tokstart != NULL && tokstart->Tok != Ord (eot_tok)) {
 	  /* we got the end of file from an abort */
-	  if (!strcmp(Yap_ErrorMessage,"Abort")) {
+	  if (Yap_ErrorMessage &&
+	      strcmp(Yap_ErrorMessage,"Abort")) {
 	    Yap_clean_tokenizer(tokstart, Yap_VarTable, Yap_AnonVarTable);
 	    return FALSE;
 	  }