Arg format: use right format for s#

s# requires Py_ssize_t for the length. Using an unsigned int results
in a corrupted stack on 64bits arch.

It was mentioned in #55

Author: GwendalRaoul

src/python4yahdlc.c

@@ -20,7 +20,8 @@ static PyObject *get_data(PyObject *self, PyObject *args)
     int ret;
     const char *frame_data;
     char recv_data[TOTAL_FRAME_LENGTH];
-    unsigned int buf_length = 0, recv_length = 0;
+    Py_ssize_t buf_length = 0;
+    unsigned int recv_length = 0;
     yahdlc_control_t control;
 
     if (!PyArg_ParseTuple(args, "s#", &frame_data, &buf_length))
@@ -89,7 +90,8 @@ static PyObject *frame_data(PyObject *self, PyObject *args)
     int ret;
     const char *send_data;
     char frame_data[TOTAL_FRAME_LENGTH];
-    unsigned int data_length = 0, frame_length = 0, frame_type = YAHDLC_FRAME_DATA, seq_no = 0;
+    Py_ssize_t data_length = 0;
+    unsigned int frame_length = 0, frame_type = YAHDLC_FRAME_DATA, seq_no = 0;
     yahdlc_control_t control;
 
     if (!PyArg_ParseTuple(args, "s#|II", &send_data, &data_length, &frame_type, &seq_no))