giochannel: Clarify assertions in g_io_channel_write_chars()

How the assertions handled the case of buf != NULL && count == -1 and buf == NULL && count == -1 were a bit fragile.

In the former case, the strlen (buf) was assigned to count, which is signed. If, somehow, buf was huge, count would end up wrapping around to a negative number. Avoid that by assigning directly to count_unsigned.

In the latter case, count_unsigned would be set to -1 which would wrap around. The error would then be caught by the precondition on buf != NULL, but it seems like that could have been a happy accident rather than something intentional. Change it to an explicit precondition which only allows buf == NULL iff count == 0.

Spotted while reading through static analysis issues, although the analyser didn’t explicitly flag this up as an issue.

Signed-off-by: Philip Withnall pwithnall@endlessos.org