BUG: Fix small memory access errors in ujson objToJSON.c

These were found (and validated fixed) using valgrind. It is likely
that none of these changes should create any issues (except maybe
on a debug build of python?). But silencing valgrind warnings is
good on its own, to ease possible future debugging using valgrind.

Author: seberg

pandas/_libs/src/ujson/python/objToJSON.c

@@ -1458,9 +1458,9 @@ char **NpyArr_encodeLabels(PyArrayObject *labels, PyObjectEncoder *enc,
 
         if (is_datetimelike) {
             if (nanosecVal == get_nat()) {
-                len = 5; // TODO: shouldn't require extra space for terminator
-                cLabel = PyObject_Malloc(len);
-                strncpy(cLabel, "null", len);
+                len = 4;
+                cLabel = PyObject_Malloc(len + 1);
+                strncpy(cLabel, "null", len + 1);
             } else {
                 if (enc->datetimeIso) {
                     if ((type_num == NPY_TIMEDELTA) || (PyDelta_Check(item))) {
@@ -1486,23 +1486,22 @@ char **NpyArr_encodeLabels(PyArrayObject *labels, PyObjectEncoder *enc,
                 }
             }
         } else { // Fallback to string representation
-            PyObject *str = PyObject_Str(item);
-            if (str == NULL) {
-                Py_DECREF(item);
+            // Replace item with the string to keep it alive.
+            Py_SETREF(item, PyObject_Str(item));
+            if (item == NULL) {
                 NpyArr_freeLabels(ret, num);
                 ret = 0;
                 break;
             }
 
-            cLabel = (char *)PyUnicode_AsUTF8(str);
-            Py_DECREF(str);
+            cLabel = (char *)PyUnicode_AsUTF8(item);
             len = strlen(cLabel);
         }
 
-        Py_DECREF(item);
         // Add 1 to include NULL terminator
         ret[i] = PyObject_Malloc(len + 1);
         memcpy(ret[i], cLabel, len + 1);
+        Py_DECREF(item);
 
         if (is_datetimelike) {
             PyObject_Free(cLabel);

pandas/tests/series/test_dtypes.py

@@ -387,10 +387,6 @@ def test_astype_generic_timestamp_no_frequency(self, dtype, request):
         data = [1]
         s = Series(data)
 
-        if np.dtype(dtype).name not in ["timedelta64", "datetime64"]:
-            mark = pytest.mark.xfail(reason="GH#33890 Is assigned ns unit")
-            request.node.add_marker(mark)
-
         msg = (
             fr"The '{dtype.__name__}' dtype has no unit\. "
             fr"Please pass in '{dtype.__name__}\[ns\]' instead."