file-string-length stream object => length
stream---an output character file stream.
object---a string or a character.
length---a non-negative integer, or nil.
file-string-length returns the difference between what (file-position stream) would be after writing object and its current value, or nil if this cannot be determined.
The returned value corresponds to the current state of stream at the time of the call and might not be the same if it is called again when the state of the stream has changed.