#4169 spmod bug when algebraic is true

[Barton Willis]

Here is a bug that manifests when gcd = spmod and algebraic = true. Define p and dp

(%i1)   p : gfactor(-((z-1)*(2*z-%i-2)^2));
(%o1)   -(z-1)*(2*z-%i-2)^2

(%i2)   dp : gfactor(diff(p,z));
(dp)    -(2*z-%i-2)*(6*z-%i-6)

Both are OK

(%i3)   block([gcd : subres, algebraic : false], gcd(p,dp));
(%o3)   2*z-%i-2

(%i4)   block([gcd : subres, algebraic : true], gcd(p,dp));
(%o4)   2*z-%i-2

Wrong for gcd = spmod and algebraic = true

(%i5)   block([gcd : spmod, algebraic : true], gcd(p,dp));
(%o5)   1

But OK for gcd = spmod and algebraic = false

(%i6)   block([gcd : spmod, algebraic : false], gcd(p,dp));
(%o6)   2*z-%i-2