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.

Also same fix in second place where code duplicates

Even an example is provided, the latter thrice as error does not arise every
time.

This fixes gap-system#2586

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;
@@ -2851,14 +2850,14 @@ InstallGlobalFunction("SubgroupsTrivialFitting",function(G)
 	tom:=TomDataAlmostSimpleRecognition(i);
 	if tom<>fail then
 	  go:=ImagesSource(tom[1]);
-	  tom:=tom[2];
-	  if tom<>fail and
+	  if tom[2]<>fail and
 	   ValueOption(NO_PRECOMPUTED_DATA_OPTION)<>true then
 	    Info(InfoPerformance,2,"Using Table of Marks Library");
 	    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)));
 	    sub:=List(sub,x->ConjugacyClassSubgroups(go,x));
 	    SetConjugacyClassesSubgroups(go,sub);
 	  fi;
@@ -2869,7 +2868,6 @@ InstallGlobalFunction("SubgroupsTrivialFitting",function(G)
 	fi;
 	Add(gold,go);
 
-
 	p:=Length(types);
       fi;
       Add(iso,IsomorphismGroups(i,gold[p]));

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

@@ -0,0 +1,12 @@
+# verify fix for bug #2586 on GitHub
+
+gap> g:=TransitiveGroup(6,14);;
+gap> cl:=ConjugacyClassesSubgroups(g);;
+gap> ForAll(cl,x->IsSubset(g,Representative(x)));
+true
+gap> Length(ConjugacyClassesSubgroups(SmallGroup(120,5)));
+12
+gap> Length(ConjugacyClassesSubgroups(SmallGroup(120,5)));
+12
+gap> Length(ConjugacyClassesSubgroups(SmallGroup(120,5)));
+12