In cli_write, the line 

int mpx = MAX(cli->max_mux-1, 1);

fails for cli->max_mux==0 due to max_mux being type size_t (unsigned)
The result is mpx set to -1, and (issued - received < mpx) is always false -
preventing cli_issue_write from ever being called.

To fix:
int mpx = (cli->max_mux > 2) ? cli->max_mux-1 : 1;

I discovered this trying to get samba to work with my Intel print station, which
requires the use of splopen/splclose as opposed to open/close, but doesn't seem
to require splwrite.  I'll submit a full patch for that shortly.