discriminant throwing an error when it probably shouldn't

typical example:
```
i1 : R=ZZ/2[x]

o1 = R

o1 : PolynomialRing

i2 : F=x^24+x^22+x^20+x^14+x^12+x^8+x^4

      24    22    20    14    12    8    4
o2 = x   + x   + x   + x   + x   + x  + x

o2 : R

i3 : discriminant(F,x)
stdio:3:12(3): error: expected nonzero polynomials
```
you can replace `ZZ/2` with any other `ZZ/p` and exponents divisible by `p`.