Reset nmod_poly_factor_t before writing to it

[fagu]

Some of the functions in nmod_poly_factor (sometimes) appended factors to the given factorization instead of replacing it. For example:

#include <cassert>
#include <flint/nmod_poly.h>
#include <flint/nmod_poly_factor.h>

int main() {
	nmod_poly_t f;
	nmod_poly_init(f, 5);
	nmod_poly_set_coeff_ui(f, 1, 1);
	nmod_poly_factor_t fac;
	nmod_poly_factor_init(fac);
	nmod_poly_factor_squarefree(fac, f);
	assert(fac->exp[0] == 1); // succeeds
	nmod_poly_factor_squarefree(fac, f);
	assert(fac->exp[0] == 1); // fails
	return 0;
}

The implementation of nmod_poly_factor_kaltofen_shoup assumed that nmod_poly_factor_distinct_deg appends the factors. I think either the documentation or the implementation of nmod_poly_factor_distinct_deg should be changed.

For nmod_poly_factor_equal_deg, I think a reasonable interpretation of the documentation is that the factors are appended, so I didn't change the behavior.