Don't call MaxSubIntersections if IdGroup is not available

[stertooy]

Context: gap-system/gap#2434

The function MaxSubIntersections currently assumes that IdGroup is available whenever it is called. And the function PGCharSubgroups calls MaxSubIntersections( G ) if G has size at most 512, so everything works fine if the SmallGrp package is loaded... but this causes an error when it's not.

This PR adds a check to see if IdGroup really is available, in addition to the size constraint.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.65%. Comparing base (dab4d91) to head (621341c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #19   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files          13       13           
  Lines        1745     1746    +1     
=======================================
+ Hits         1547     1548    +1     
  Misses        198      198           

Files with missing lines	Coverage Δ
gap/initmat.gi	100.00% <100.00%> (ø)

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