In fact I can simplify the expression to

a(n)=sum(a=0,n, sum(b=0,a, my(C,c)))

for(n=0,1e7,a(n))

while still segfaulting with the same behavior (Ctrl+C, type n, press enter), while replacing the function with either

does not segfault as far as I can tell.

The variable names do not seems to matter; a(n)=sum(a=0,n,sum(b=0,a,my(x,y))) behaves the same way.