avoid segmentation faults in universe/uloop variable increment

  } else if (istyle == UNIVERSE || istyle == ULOOP) {

    uloop_again:

    // wait until lock file can be created and owned by proc 0 of this world

    // rename() is not atomic in practice, but no known simple fix

    //   means multiple procs can read/write file at the same time (bad!)

    // delay for random fraction of 1 second before subsequent tries

    // when successful, read next available index and Bcast it within my world

    int nextindex;

    int nextindex = -1;

    if (me == 0) {

      int seed = 12345 + universe->me + which[find(arg[0])];

      RanMars *random = new RanMars(lmp,seed);

      }

      delete random;

      FILE *fp = fopen("tmp.lammps.variable.lock","r");

      fscanf(fp,"%d",&nextindex);

      //printf("READ %d %d\n",universe->me,nextindex);

      // if the file cannot be found, we may have a race with some

      // other MPI rank that has called rename at the same time

      // and we have to start over.

      // if the read is short (we need at least one byte) we try reading again.

      FILE *fp;

      char buf[64];

      for (int loopmax = 0; loopmax < 100; ++loopmax) {

        fp = fopen("tmp.lammps.variable.lock","r");

        if (fp == NULL) goto uloop_again;

        buf[0] = buf[1] = '\0';

        fread(buf,1,64,fp);

        fclose(fp);

        if (strlen(buf) > 0) {

           nextindex = atoi(buf);

           break;

        }

        delay = (int) (1000000*random->uniform());

        usleep(delay);

      }

      if (nextindex < 0)

        error->one(FLERR,"Unexpected error while incrementing uloop "

                   "style variable. Please contact LAMMPS developers.");

      //printf("READ %d %d\n",universe->me,nextindex);

      fp = fopen("tmp.lammps.variable.lock","w");

      fprintf(fp,"%d\n",nextindex+1);

      //printf("WRITE %d %d\n",universe->me,nextindex+1);