An/Sn: guard against inputs outside the group

[fingolfin]

Extracted from PR #428. Ideally would have a dedicated test (i.e. trying to test membership in An for outsider elements), but then again PR #428 of course also contains a test for the underlying issue...

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (0acc6e4) to head (d30356f).

Files with missing lines	Patch %	Lines
gap/perm/giant.gi	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   88.18%   88.17%   -0.01%     
==========================================
  Files          45       45              
  Lines       19004    19010       +6     
==========================================
+ Hits        16759    16763       +4     
- Misses       2245     2247       +2     

Files with missing lines	Coverage Δ
gap/perm/giant.gi	94.84% <66.66%> (-0.37%)	⬇️

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