add thorough checking for valid arguments to -partition or -p

  int n,nper;

  char *ptr;

  if (str == NULL) {

  n = 1;

    nper = nprocs;

  } else if ((ptr = strchr(str,'x')) != NULL) {

  if (str != NULL) {

    // check for valid partition argument

    bool valid = true;

    // str may not be empty and may only consist of digits or 'x'

    int len = strlen(str);

    if (len < 1) valid = false;

    for (int i=0; i < len; ++i)

      if (isdigit(str[i]) || str[i] == 'x') continue;

      else valid = false;

    if (valid) {

      if ((ptr = strchr(str,'x')) != NULL) {

        // 'x' may not be the first or last character

        if (ptr == str) {

          valid = false;

        } else if (strlen(str) == len-1) {

          valid = false;

        } else {

          *ptr = '\0';

          n = atoi(str);

          nper = atoi(ptr+1);

  } else {

    n = 1;

    nper = atoi(str);

          *ptr = 'x';

        }

      } else nper = atoi(str);

    }

    // require minimum of 1 partition with 1 processor

    if (n < 1 || nper < 1) valid = false;

    if (!valid) {

      char msg[128];

      sprintf(msg,"Invalid partition string '%s'",str);

      error->universe_all(FLERR,msg);

    }

  } else nper = nprocs;

  memory->grow(procs_per_world,nworlds+n,"universe:procs_per_world");

  memory->grow(root_proc,(nworlds+n),"universe:root_proc");