json: Fix parsing of strings that end with a backslash.

json_string_unescape() flagged a backslash at the end of a string as an
error, but of course "\\" is a valid string.  This fixes the problem.

VMware-BZ: #1275208
Reported-by: Michael Hu <mhu@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>