Incomplete string input on command line freezes SITL

In SITL:

If I input a ' character at the MAVProxy prompt (or a command with an accidental trailing '), ArduPlane ceases accepting commands and the map and console become frozen. Error at MAVProxy is:

MANUAL> '
MANUAL> Exception in thread main_loop:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
self.run()
File "/usr/lib/python2.7/threading.py", line 763, in run
self.__target(_self.__args, *_self.__kwargs)
File "/usr/local/bin/mavproxy.py", line 731, in main_loop
process_stdin(c)
File "/usr/local/bin/mavproxy.py", line 447, in process_stdin
args = shlex.split(line)
File "/usr/lib/python2.7/shlex.py", line 279, in split
return list(lex)
File "/usr/lib/python2.7/shlex.py", line 269, in next
token = self.get_token()
File "/usr/lib/python2.7/shlex.py", line 96, in get_token
raw = self.read_token()
File "/usr/lib/python2.7/shlex.py", line 172, in read_token
raise ValueError, "No closing quotation"
ValueError: No closing quotation

Start in mavproxy.py at process_stdin, where the traceback shows shlex.split(line) failing on an unmatched quote. Reproduce the input in SITL and verify that MAVProxy continues accepting commands and that the map and console remain responsive afterward.

Written by the indexing model from the issue text.