Fix JSON::ResumableParser#parse leaking the in_use lock on an empty buffer

[mame]

#parse sets in_use then returns early on an empty buffer without clearing it, permanently bricking the parser.

require "json"
parser = JSON::ResumableParser.new
parser.parse          # => false, but leaks in_use = true
parser << "[1,2,3]"
parser.parse          # ArgumentError: ResumableParser can't be used recursively