[luatex] Odd Lua issue with ^^@

[Akira Kakuto]

> ....\T1/cmr/m/n/10 ^@
> ....\kern -5.13763 (for accent)

Previously we were using static int read_line() by Taco.
Now we are using one in lua 5.2.4, where there is a line:
l = strlen(p);
Probably this seems to be a reason of the difference.

Best,
Akira