Avoid calling ConvertToVectorRep(v, 0)

[frankluebeck]

The family of finite fields of size >= 256 sets
!.rchar = 0, which leads to calls of 'ConvertToVectorRep'
with second argument 0 in various places.

Added a test which demonstrates the problem.

Description

The problem is described above.

The bug addressed here can be seen in this example:

gap> pol := UnivariatePolynomial(GF(293), Z(293)^0 * ConwayPol(293,8));;
gap> K := AlgebraicExtension(GF(293), pol);;
gap> xinv := 1/PrimitiveElement(K);
Error, <n> must be a positive integer at /export/home/luebeck/gap4/lib/integer.gi:991 called from
LogInt( q, common ) at /export/home/luebeck/gap4/lib/vecmat.gi:1365 called from
ConvertToVectorRepNC( v, f ); at /export/home/luebeck/gap4/lib/vecmat.gi:1530 called from
ImmutableVector( fam!.rchar, rf, true 
 ) at /export/home/luebeck/gap4/lib/algfld.gi:611 called from
InverseOp( elm ) at /export/home/luebeck/gap4/lib/arith.gi:392 called from
<function "InverseSameMutability for an (immutable) object">( <arguments> )
 called from read-eval loop at *stdin*:3
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> 
gap> c := Random(K);;
Error, <n> must be a positive integer at /export/home/luebeck/gap4/lib/integer.gi:991 called from
LogInt( q, common ) at /export/home/luebeck/gap4/lib/vecmat.gi:1365 called from
ConvertToVectorRepNC( v, f ); at /export/home/luebeck/gap4/lib/vecmat.gi:1530 called from
ImmutableVector( fam!.rchar, l, true 
 ) at /export/home/luebeck/gap4/lib/algfld.gi:910 called from
func( GlobalMersenneTwister, x 
 ) at /export/home/luebeck/gap4/lib/random.gi:299 called from
<function "Random for an algebraic extension">( <arguments> )
 called from read-eval loop at *stdin*:3
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> 

I'm not sure, if this pull request catches all instances of the problem. It is at least a quick fix which should never be bad.

A better fix would probably try to avoid that for larger finite fields its family has a component !.rchar which is set to 0 (instead of the characteristic of the field).

Text for release notes

A bug in the arithmetic of larger finite fields was fixed.

[fingolfin]

Whenever lib/vecmat.gi is modified, the same changes need to be applied to hpcgap/hpcgap/lib/vecmat.gi.

I'll submit an alternative PR.

[fingolfin]

I don't think we should accept 0 here, that's a usage error; instead, the calling code should be fixed; we could insert an assertion here, though, to catch further similar issues in the future.