Fix JSON::ResumableParser stalling on a NUL byte

[mame]

Feeding "[\x00]" makes #parse return false forever and never make progress.

require "json"
parser = JSON::ResumableParser.new
parser << "[\x00]"
parser.parse   # => false
parser.parse   # => false
parser.parse   # => false (forever)

Fix: a NUL byte that is not at the end of the stream is treated as a syntax error (it can never become valid by feeding more), as the non-resumable parser already does.