ellgroup hangs for certain input on Version 2.6.2 (development git-0f10e3b)

[Richard Heylen <richardheylen@gmail.com>]

I believe it's policy that functions don't bother to check for bad
input values in order to keep things fast. However, when a function
hangs using 100% CPU on bad input then I think it needs to be fixed
because this would be bad as part of a long running cpu intensive pari
script.

Cheers!

Richard

? aa=ffgen(Mod(1,3)*x^2+Mod(2,3)*x+Mod(1,3),'a)
%1 = a
? e=ellinit([aa^3,0],aa)
%2 = [0, 0, 0, 2, 0, 0, 1, 0, 2, 0, 0, 1, 0, Vecsmall([4]), [a,
[Vecsmall([70368744177664, 2]), Vecsmall([70368744177664]),
[Vecsmall([70368744177664, 1]), Vecsmall([70368744177664]),
Vecsmall([70368744177664]), Vecsmall([70368744177664])]]], [0, 0, 0,
0]]
? ellgroup(e)