parseint_preamble bounds check fails for AbstractString

[samoconnor]

i <= ncodeunits(s) should be i <= endpos ?

julia/base/parse.jl

Lines 75 to 76 in 1a1d6b6

	if c == '0' && i <= ncodeunits(s)
	c, i = iterate(s,i)::Tuple{Char, Int}

When I do parse(Int, "0") where "0" is an AbstractString and ncodeunits > lastindex, i get this:
TypeError: in parseint_preamble, in typeassert, expected Tuple{Char,Int64}, got Nothing

1a1d6b6#r30713661

Monkeypatching parseint_preamble for my string type with i <= endpos seems to fix the problem.

[StefanKarpinski]

Since there seems to be a fix in that monkey patch, would you be willing to make a PR to fix this in Base Julia?