Fix hang on macOS

[fingolfin]

Kudos to @ChrisJefferson who discovered this fix.

Resolves #47 Closes #52

[codecov[bot]]

Codecov Report

Merging #53 (d915652) into master (6da2a56) will decrease coverage by 7.14%. The diff coverage is 12.50%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   48.04%   40.89%   -7.15%     
==========================================
  Files         147      147              
  Lines       12886    12873      -13     
  Branches     1940     1941       +1     
==========================================
- Hits         6191     5265     -926     
- Misses       6250     7198     +948     
+ Partials      445      410      -35     

Impacted Files	Coverage Δ
src/pretty_filter.c	42.64% <12.50%> (-1.04%)	:arrow_down:
src/find_allowable_subgroup.c	0.00% <0.00%> (-85.72%)	:arrow_down:
src/standard.c	0.00% <0.00%> (-83.04%)	:arrow_down:
src/collect.c	7.73% <0.00%> (-80.12%)	:arrow_down:
src/invert_modp.c	37.50% <0.00%> (-62.50%)	:arrow_down:
src/isom_options.c	9.23% <0.00%> (-54.18%)	:arrow_down:
src/collect_word.c	11.76% <0.00%> (-41.18%)	:arrow_down:
src/find_permutation.c	63.63% <0.00%> (-36.37%)	:arrow_down:
src/map_relations.c	0.00% <0.00%> (-31.32%)	:arrow_down:
src/echelonise_matrix.c	36.36% <0.00%> (-27.28%)	:arrow_down:
... and 35 more