Fix NULL pointer dereference in read_post_data()

What took down cvs/sbbs yesterday:
Program terminated with signal SIGSEGV, Segmentation fault.
6203                    session->req.post_data[session->req.post_len]=0;
[Current thread is 1 (Thread 0x7f2b989ff700 (LWP 17031))]
(gdb) print post_len
No symbol "post_len" in current context.
(gdb) print session->req.post_len
$1 = 0
(gdb) print session->req.post_data
$2 = 0x0