Fix ShortestVector (#941)

* Fix an edge case in ShortestVectors when an empty list could be left when "positive" is given as a parameter to ShortestVectors.
* Add a test.

Closes: #838 #839

lib/zlattice.gi

@@ -1331,7 +1331,7 @@ InstallGlobalFunction( ShortestVectors, function( arg )
        c.vectors[anz][i] := w;
     od;
     if checkpositiv and neg then
-       c.vectors[anz] := [];
+       Unbind(c.vectors[anz]);
        anz := anz - 1;
     else
        c.norms[anz] := dam;

tst/teststandard/bugfix.tst

@@ -3155,6 +3155,16 @@ gap> g:=Group((1,2,4,3)(5,7,6,8)(9,11,10,13)(12,16,14,15)(17,19,18,20)
 gap> Size(FrattiniSubgroup(g));
 2
 
+# 2016/11/08 (MP), #838, output of OrthognalEmbeddings is very long
+gap> g := [ [ 6, 2, 2, 2, 2, 2, 0, 0, 0, 0 ], [ 2, 4, 0, 0, 0, 0, 0, 0, 0, 0 ], [ 2, 0, 4, 0, 0, 0, 2, 0, 0, 0 ], 
+> [ 2, 0, 0, 4, 0, 0, 0, 2, 0, 0 ], [ 2, 0, 0, 0, 4, 0, 0, 0, 2, 0 ], [ 2, 0, 0, 0, 0, 4, 0, 0, 0, 2 ], 
+> [ 0, 0, 2, 0, 0, 0, 6, 0, 0, 0 ], [ 0, 0, 0, 2, 0, 0, 0, 6, 0, 0 ], [ 0, 0, 0, 0, 2, 0, 0, 0, 6, 0 ], 
+> [ 0, 0, 0, 0, 0, 2, 0, 0, 0, 6 ] ];;
+gap> oe := OrthogonalEmbeddings(g, "positive");;
+gap> Length(oe.vectors); Length(oe.solutions);
+159
+0
+
 #############################################################################
 gap> STOP_TEST( "bugfix.tst", 831990000);