remove needs some sanitization

[tthsqe12]

julia> R, z = FiniteField(fmpz(7), 5, "z")
julia> Rx, x = PolynomialRing(R, "x")
julia> remove(x, zero(Rx))

signal (11): Segmentation fault
in expression starting at REPL[5]:1
fq_inv at /workspace/srcdir/flint2-2.8.0/fq/inv.c:36
unknown function (ip: 0x7ff7c9802c4f)
unknown function (ip: (nil))
Allocations: 4365692 (Pool: 4364293; Big: 1399); GC: 4
Segmentation fault (core dumped)

[wbhart]

Sure, but the docs do explicitly say if you try to remove the zero, the entire universe explodes. We should at least try to give people a feel for what we mean by that. :-)