Prevents overflow on long lines, which could result in a negative
number being returned even if the read was otherwise successful.

Change the type of len, i and b so they can index positions
over 2Gbytes into the input string.  Naturally this is only useful
on 64 bit platforms as the entire input needs to be in memory.

Luckily this function isn't exported, so changing the type of
len does not affect the ABI.

It needs to be ssize_t to prevent possible overflow when writing
very long records on 64 bit platforms.