pgcd(): avoid infinite loop if the first GCD candidate coincides with GCD

pgcd(): avoid infinite loop if the first GCD candidate coincides with GCD

136	if (H_lcoeff.is_equal(lc_gcd)) {
137		if ((Hprev-H).expand().smod(pn).is_zero()) // (*)
138			continue;

The check (*) can result in infinite loop if the (very) first GCD candidate
gives the correct GCD: any subsequent iterations give the same result, so
Hprev and H will be equal (hence the infinite loop). That check is not very
useful (AFAIR I was trying to avoid extra division checks), so let's remove it.