Prevent ResumableParser from stalling on invalid object keys

[ydah]

A backslash followed by a literal NUL in an object key makes ResumableParser keep returning false, even after more input arrives.

require "json"

parser = JSON::ResumableParser.new
parser << "{\"key\\\x00\":1}"
parser.parse
# Before: false on every retry
# After: raises JSON::ParserError

Only rewind and wait when the cursor has reached the end of input, matching string-value parsing.