bpo-34824: Fix a possible NULL pointer dereference in _ssl.c (GH-9606)

On failure, _PyBytes_Resize() will deallocate the bytes object and set
"result" to NULL.


https://bugs.python.org/issue34824

Misc/NEWS.d/next/Core and Builtins/2018-09-27-11-10-02.bpo-34824.VLlCaU.rst

+Fix a possible null pointer dereference in Modules/_ssl.c. Patch by Zackery
+Spytz.

Modules/_ssl.c

@@ -4710,12 +4710,17 @@ _ssl_MemoryBIO_read_impl(PySSLMemoryBIO *self, int len)
         return result;
 
     nbytes = BIO_read(self->bio, PyBytes_AS_STRING(result), len);
-    /* There should never be any short reads but check anyway. */
-    if ((nbytes < len) && (_PyBytes_Resize(&result, len) < 0)) {
+    if (nbytes < 0) {
         Py_DECREF(result);
+        _setSSLError(NULL, 0, __FILE__, __LINE__);
         return NULL;
     }
 
+    /* There should never be any short reads but check anyway. */
+    if (nbytes < len) {
+        _PyBytes_Resize(&result, nbytes);
+    }
+
     return result;
 }