gh-105375: Improve error handling in the sys extension module (#105611)

In _PySys_AddXOptionWithError() and sys_add_xoption(),
bail on first error to prevent exceptions from possibly being
overwritten.

Author: erlend-aasland

Misc/NEWS.d/next/Library/2023-06-09-23-46-23.gh-issue-105375.9KaioS.rst

@@ -0,0 +1,2 @@
+Fix bugs in :mod:`sys` where exceptions could end up being overwritten
+because of deferred error handling.

Python/sysmodule.c

@@ -2721,14 +2721,20 @@ _PySys_AddXOptionWithError(const wchar_t *s)
     const wchar_t *name_end = wcschr(s, L'=');
     if (!name_end) {
         name = PyUnicode_FromWideChar(s, -1);
+        if (name == NULL) {
+            goto error;
+        }
         value = Py_NewRef(Py_True);
     }
     else {
         name = PyUnicode_FromWideChar(s, name_end - s);
+        if (name == NULL) {
+            goto error;
+        }
         value = PyUnicode_FromWideChar(name_end + 1, -1);
-    }
-    if (name == NULL || value == NULL) {
-        goto error;
+        if (value == NULL) {
+            goto error;
+        }
     }
     if (PyDict_SetItem(opts, name, value) < 0) {
         goto error;
@@ -3374,14 +3380,20 @@ sys_add_xoption(PyObject *opts, const wchar_t *s)
     const wchar_t *name_end = wcschr(s, L'=');
     if (!name_end) {
         name = PyUnicode_FromWideChar(s, -1);
+        if (name == NULL) {
+            goto error;
+        }
         value = Py_NewRef(Py_True);
     }
     else {
         name = PyUnicode_FromWideChar(s, name_end - s);
+        if (name == NULL) {
+            goto error;
+        }
         value = PyUnicode_FromWideChar(name_end + 1, -1);
-    }
-    if (name == NULL || value == NULL) {
-        goto error;
+        if (value == NULL) {
+            goto error;
+        }
     }
     if (PyDict_SetItem(opts, name, value) < 0) {
         goto error;