[3.6] bpo-30769: Fix reference leak introduced in 77703942 (GH-2416) (#2425)

New error condition paths were introduced, which did not decrement
`key2` and `val2` objects.  Therefore, decrement references before
jumping to the error label.
Signed-off-by: Eric N. Vander Weele <ericvw@gmail.com>
(cherry picked from commit a7874c73)

Misc/ACKS

@@ -1590,6 +1590,7 @@ Andi Vajda
 Case Van Horsen
 John Mark Vandenberg
 Kyle VanderBeek
+Eric N. Vander Weele
 Andrew Vant
 Atul Varma
 Dmitry Vasiliev

Modules/posixmodule.c

@@ -4823,6 +4823,8 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
             PyUnicode_FindChar(key2, '=', 1, PyUnicode_GET_LENGTH(key2), 1) != -1)
         {
             PyErr_SetString(PyExc_ValueError, "illegal environment variable name");
+            Py_DECREF(key2);
+            Py_DECREF(val2);
             goto error;
         }
         keyval = PyUnicode_FromFormat("%U=%U", key2, val2);
@@ -4837,6 +4839,8 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
             strchr(PyBytes_AS_STRING(key2) + 1, '=') != NULL)
         {
             PyErr_SetString(PyExc_ValueError, "illegal environment variable name");
+            Py_DECREF(key2);
+            Py_DECREF(val2);
             goto error;
         }
         keyval = PyBytes_FromFormat("%s=%s", PyBytes_AS_STRING(key2),