added TmpOutFile=NULL; after closing TmpOutFile

In case of [TIMES] Statistic not equal NONE (see page 174 of Epanet2 users manual) ENclose() function tries to close TmpOutFile again giving a "double free or corruption" error

src/output.c

@@ -503,6 +503,7 @@ int  savefinaloutput()
       ERRCODE(savetimestat(x,LINKHDR));
       if (!errcode) Nperiods = 1;
       fclose(TmpOutFile);
+      TmpOutFile=NULL;
       free(x);
    }