FIX: Pull subgroups of TOM for lattice back through isomorphism

This bug likely did not show up earlier since the TOM library was not loaded
by default.

This partially fixes #2586
(main bug reported there)

lib/grplatt.gi

@@ -2819,11 +2819,10 @@ InstallGlobalFunction("SubgroupsTrivialFitting",function(G)
 	ValueOption(NO_PRECOMPUTED_DATA_OPTION)<>true then
       Info(InfoPerformance,2,"Using Table of Marks Library");
       go:=ImagesSource(tom[1]);
-      tom:=tom[2];
       Info(InfoLattice,1, "Fetching subgroups of simple ",
-	  Identifier(tom)," from table of marks");
-      len:=LengthsTom(tom);
-      sub:=List([1..Length(len)],x->RepresentativeTom(tom,x));
+	  Identifier(tom[2])," from table of marks");
+      len:=LengthsTom(tom[2]);
+      sub:=List([1..Length(len)],x->PreImage(tom[1],RepresentativeTom(tom[2],x)));
       return sub;
     fi;
   fi;

tst/testbugfix/2018-06-28-grplatt.tst

@@ -0,0 +1,7 @@
+# #2586
+
+gap> g:=TransitiveGroup(6,14);;
+gap> cl:=ConjugacyClassesSubgroups(g);;
+gap> ForAll(cl,x->IsSubset(g,Representative(x)));
+true
+