bpo-6393: Fix locale.getprerredencoding() on macOS (#1555)

Fix for bpo-6393: Python crashes on OSX when $LANG is set to some (but
not all) invalid values due to an invalid result from nl_langinfo

(cherry picked from commit 6d77e07)

Author: vstinner

Lib/locale.py

@@ -617,10 +617,22 @@ def getpreferredencoding(do_setlocale = True):
                 except Error:
                     pass
                 result = nl_langinfo(CODESET)
+                if not result and sys.platform == 'darwin':
+                    # nl_langinfo can return an empty string
+                    # when the setting has an invalid value.
+                    # Default to UTF-8 in that case because
+                    # UTF-8 is the default charset on OSX and
+                    # returning nothing will crash the
+                    # interpreter.
+                    result = 'UTF-8'
+
                 setlocale(LC_CTYPE, oldloc)
                 return result
             else:
-                return nl_langinfo(CODESET)
+                result = nl_langinfo(CODESET)
+                if not result and sys.platform == 'darwin':
+                    # See above for explanation
+                    result = 'UTF-8'
 
 
 ### Database