bpo-36763: Fix Py_SetStandardStreamEncoding() (GH-13028)

Fix memory leak in Py_SetStandardStreamEncoding(): release memory
if the function is called twice.

Misc/NEWS.d/next/C API/2019-05-01-00-42-08.bpo-36763.vghb86.rst

+Fix memory leak in :c:func:`Py_SetStandardStreamEncoding`: release memory if
+the function is called twice.

Python/coreconfig.c

@@ -375,6 +375,7 @@ Py_SetStandardStreamEncoding(const char *encoding, const char *errors)
      * Py_Initialize hasn't been called yet.
      */
     if (encoding) {
+        PyMem_RawFree(_Py_StandardStreamEncoding);
         _Py_StandardStreamEncoding = _PyMem_RawStrdup(encoding);
         if (!_Py_StandardStreamEncoding) {
             res = -2;
@@ -382,11 +383,11 @@ Py_SetStandardStreamEncoding(const char *encoding, const char *errors)
         }
     }
     if (errors) {
+        PyMem_RawFree(_Py_StandardStreamErrors);
         _Py_StandardStreamErrors = _PyMem_RawStrdup(errors);
         if (!_Py_StandardStreamErrors) {
-            if (_Py_StandardStreamEncoding) {
             PyMem_RawFree(_Py_StandardStreamEncoding);
-            }
+            _Py_StandardStreamEncoding = NULL;
             res = -3;
             goto done;
         }