Change growable string object to null terminate

Change growable string object to null terminate

src/ChangeLog
src/url.c

@ src/url.c:

Any function directly writing to the members of a growable string
object must call append_null(). Functions that already do this I've
changed to call append_null(): shorten_length(), append_char(),
append_string(), append_uri_pathel().

- (append_null)
New function to null terminate a growable string object.

- (shorten_length)
New function to shorten the length of a growable string object.