bug in sum() and probably others

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Hello PARI Dev,

On a x86,

? sum(i=2^32-10,2^32+10,1)
%1 = 11

Looking at the code, it seems there are several bugs
here. incpos() does not use the t_INT interface and
is probably not memory-safe, and the gerepile() call
does not protect a.

Cheers,
Bill.