avoid potentially unsafe downcast

Cython/Utility/StringTools.c

@@ -395,23 +395,19 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
 //@requires: IncludeStringH
 
 /* duplicate code to avoid calling strlen() if start >= 0 and stop >= 0 */
-/* Casting to Py_ssize_t and checking for negative values to determine
- * if overflow occured should be safe in practice (it's undefined
- * behavior by C99, but common behavior for systems with
- * two-complement representation).
- */
 static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
          const char* cstring, Py_ssize_t start, Py_ssize_t stop,
          const char* encoding, const char* errors,
          PyObject* (*decode_func)(const char *s, Py_ssize_t size, const char *errors)) {
     Py_ssize_t length;
     if (unlikely((start < 0) | (stop < 0))) {
-        length = (Py_ssize_t) strlen(cstring);
-        if(length < 0) {
+        size_t slen = strlen(cstring);
+        if (unlikely(slen > (size_t) PY_SSIZE_T_MAX)) {
             PyErr_SetString(PyExc_OverflowError,
                             "c-string too long to convert to Python");
             return NULL;
         }
+        length = (Py_ssize_t) slen;
         if (start < 0) {
             start += length;
             if (start < 0)