protect warnings

[zkamvar]

this comes from https://github.com/kalibera/rprotect/blob/master/reports/poppr/poppr.so.bcheck

Function bruvo_distance
  [UP] unprotected variable bruvo_mat while calling allocating function Rf_coerceVector poppr/src/poppr_distance.c:229
  [UP] unprotected variable bruvo_mat while calling allocating function Rf_allocMatrix poppr/src/poppr_distance.c:230
  [UP] unprotected variable bruvo_mat while calling allocating function Rf_allocVector poppr/src/poppr_distance.c:231

Function pairwise_covar
  [UP] unprotected variable pair_vec while calling allocating function Rf_allocVector poppr/src/poppr_distance.c:86

Function permute_shuff
  [UP] unprotected variable alleles while calling allocating function Rf_coerceVector poppr/src/permut_shuffler.c:67

These should be fixed.