Infinite loop in `fmpz_poly_randtest_irreducible`

[mezzarobba]

A very minor issue tangentially related to #2140: fmpz_poly_randtest_irreducible(u, 0x7ffd631cdc50, 1, 10); seems to go into an infinite loop although, over the integers, there are plenty of irreducible polynomials satisfying the size bounds.