MemoryError raised in readline.set_completer_delims() turns the readline library to incorrect state.

$ (ulimit -v 200000; ./python;)
Python 3.6.0a0 (default:e33b4c18af59+, Sep 17 2015, 17:05:17) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import readline
>>> readline.get_completer_delims()
' \t\n`~!@#$%^&*()-=+[{]}\\|;:\'",<>/?'
>>> readline.set_completer_delims(' '*10**8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError
>>> readline.get_completer_delims()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe4 in position 1: invalid continuation byte

Proposed patch fixes the issue.