pythongh-156939: Fix struct.pack('0p', bytes) (pythonGH-157071)

If the Pascal string is empty (size=0), do not write the size prefix.
Previously, a NUL byte was written outsize the buffer (buffer
overflow). In practice, the write remains into allocated memory
and is silently ignored: no memory is corrupted.
(cherry picked from commit 23525c9)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Modules/_struct.c

@@ -2395,7 +2395,9 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args,
                     memcpy(res + 1, p, n);
                 if (n > 255)
                     n = 255;
-                *res = Py_SAFE_DOWNCAST(n, Py_ssize_t, unsigned char);
+                if (n > 0) {
+                    *res = Py_SAFE_DOWNCAST(n, Py_ssize_t, unsigned char);
+                }
             } else {
                 if (e->pack(state, res, v, e) < 0) {
                     if (PyLong_Check(v) && PyErr_ExceptionMatches(PyExc_OverflowError))