[3.11] gh-105375: Improve posix error handling (GH-105592) (#105597)

Fix a bug where an IndexError could end up being overwritten.
(cherry picked from commit f668f73)

Co-authored-by: Erlend E. Aasland <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2023-06-09-21-30-59.gh-issue-105375.eewafp.rst

@@ -0,0 +1,2 @@
+Fix a bug in the :mod:`posix` module where an exception could be
+overwritten.

Modules/posixmodule.c

@@ -5691,7 +5691,7 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
 {
     Py_ssize_t i, pos, envc;
     PyObject *keys=NULL, *vals=NULL;
-    PyObject *key, *val, *key2, *val2, *keyval;
+    PyObject *key2, *val2, *keyval;
     EXECV_CHAR **envlist;
 
     i = PyMapping_Size(env);
@@ -5716,10 +5716,14 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
     }
 
     for (pos = 0; pos < i; pos++) {
-        key = PyList_GetItem(keys, pos);
-        val = PyList_GetItem(vals, pos);
-        if (!key || !val)
+        PyObject *key = PyList_GetItem(keys, pos);  // Borrowed ref.
+        if (key == NULL) {
             goto error;
+        }
+        PyObject *val = PyList_GetItem(vals, pos);  // Borrowed ref.
+        if (val == NULL) {
+            goto error;
+        }
 
 #if defined(HAVE_WEXECV) || defined(HAVE_WSPAWNV)
         if (!PyUnicode_FSDecoder(key, &key2))