[3.7] bpo-33383: Fix crash in get() of the dbm.ndbm database object. (GH-6630) (GH-6631)

(cherry picked from commit 2e38cc39330bd7f3003652869b644110a97a78d8)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
index 8afd92c..65761d8 100644
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -274,7 +274,7 @@
 _dbm.dbm.get
 
     key: str(accept={str, robuffer}, zeroes=True)
-    default: object(c_default="NULL") = b''
+    default: object = None
     /
 
 Return the value for key if present, otherwise default.
@@ -283,7 +283,7 @@
 static PyObject *
 _dbm_dbm_get_impl(dbmobject *self, const char *key,
                   Py_ssize_clean_t key_length, PyObject *default_value)
-/*[clinic end generated code: output=b44f95eba8203d93 input=a3a279957f85eb6d]*/
+/*[clinic end generated code: output=b44f95eba8203d93 input=b788eba0ffad2e91]*/
 /*[clinic end generated code: output=4f5c0e523eaf1251 input=9402c0af8582dc69]*/
 {
     datum dbm_key, val;
diff --git a/Modules/clinic/_dbmmodule.c.h b/Modules/clinic/_dbmmodule.c.h
index 0f831c9..574cc5e 100644
--- a/Modules/clinic/_dbmmodule.c.h
+++ b/Modules/clinic/_dbmmodule.c.h
@@ -39,7 +39,7 @@
 }
 
 PyDoc_STRVAR(_dbm_dbm_get__doc__,
-"get($self, key, default=b\'\', /)\n"
+"get($self, key, default=None, /)\n"
 "--\n"
 "\n"
 "Return the value for key if present, otherwise default.");
@@ -57,7 +57,7 @@
     PyObject *return_value = NULL;
     const char *key;
     Py_ssize_clean_t key_length;
-    PyObject *default_value = NULL;
+    PyObject *default_value = Py_None;
 
     if (!_PyArg_ParseStack(args, nargs, "s#|O:get",
         &key, &key_length, &default_value)) {
@@ -141,4 +141,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=5c858b4080a011a4 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=1cba297bc8d7c2c2 input=a9049054013a1b77]*/