Changeset [8e489a46965bbc3683cec7698ccaf71bb5855d2a] by Christophe Fergeau

make sure 'length' is >=0 in afc_file_write

trying to write a negative amount of data doesn't make much sense, and bad
things will happen if we try to do that. Just treat negative length values
as an invalid argument. An alternative way of handling it would be to
silently return OK/0 bytes written.
http://github.com/MattColyer/libiphone/commit/8e489a46965bbc3683cec...