Fix segfault in Record.get() with invalid positional arg count

record_get() set a TypeError with PyErr_Format() when given the wrong
number of positional arguments but did not return, falling through to
record_item_by_name() with an uninitialized `key` pointer. Depending on
the stack contents this either crashed the interpreter (SEGV) or silently
returned the default instead of raising TypeError. The release build also
emitted a `'key' may be used uninitialized` warning.

Return NULL immediately after setting the error, and add regression
coverage for the invalid argument-count cases.

Fixes #1328.

Author: apoorvdarshan

asyncpg/protocol/record/recordobj.c

@@ -692,6 +692,7 @@ record_get(PyObject *self, PyTypeObject *defcls, PyObject *const *args,
         PyErr_Format(PyExc_TypeError,
                      "Record.get() expected 1 or 2 arguments, got %zd",
                      nargs);
+        return NULL;
     }
 
     if (kwnames != NULL && PyTuple_GET_SIZE(kwnames) != 0) {

tests/test_record.py

@@ -288,6 +288,16 @@ def test_record_get(self):
             self.assertEqual(r.get('nonexistent'), None)
             self.assertEqual(r.get('nonexistent', 'default'), 'default')
 
+    def test_record_get_invalid_args(self):
+        r = Record(R_AB, (42, 43))
+        with self.checkref(r):
+            with self.assertRaises(TypeError):
+                r.get()
+            with self.assertRaises(TypeError):
+                r.get('a', 2, 3)
+            with self.assertRaises(TypeError):
+                r.get(default=2)
+
     def test_record_not_pickleable(self):
         r = Record(R_A, (42,))
         with self.assertRaises(Exception):