Make generic SubdirectProductOp more robust

The generic SubdirectProductOp performs "recursion". But SubdirectProductOp in
general expects the mappings given to it be group homomorphisms, and also that
they "know" about this. Since it sometimes happens that mappings are produced
that do not "know" this information, explicitly check for this (just as
`SubdirectProduct` does).

Fixes #3431 "even more"

lib/gprd.gi

@@ -544,6 +544,11 @@ local gc,hc,S,info;
   fi;
   gh:=InverseGeneralMapping(gc)*gh;
   hh:=InverseGeneralMapping(hc)*hh;
+  # the ...Op is installed for `IsGroupHomomorphism'. So we have to enforce
+  # the filter to be set.
+  if not IsGroupHomomorphism(gh) or not IsGroupHomomorphism(hh) then
+    Error("mappings are not homomorphisms");
+  fi;
   S:=SubdirectProductOp(Image(gc,G),Image(hc,H),gh,hh);
   info:=rec(groups:=[G,H],
 	    homomorphisms:=[gh,hh],