Add PY_SSIZE_T_MIN, as suggested by Ralf W. Grosse-Kunstleve.

Include/pyport.h

@@ -99,6 +99,8 @@ typedef Py_intptr_t	Py_ssize_t;
 
 /* Largest positive value of type Py_ssize_t. */
 #define PY_SSIZE_T_MAX ((Py_ssize_t)(((size_t)-1)>>1))
+/* Smallest positive value of type Py_ssize_t. */
+#define PY_SSIZE_T_MIN (-PY_SSIZE_T_MAX-1)
 
 /* PY_FORMAT_SIZE_T is a platform-specific modifier for use in a printf
  * format to convert an argument with the width of a size_t or Py_ssize_t.

Objects/longobject.c

@@ -281,7 +281,7 @@ _long_as_ssize_t(PyObject *vv) {
 	if (sign > 0) 
 		return PY_SSIZE_T_MAX;
 	else 
-		return -PY_SSIZE_T_MAX-1;
+		return PY_SSIZE_T_MIN;
 }
 
 /* Get a Py_ssize_t from a long int object.