Fixed crash with sigabort on invalid surrogate sequence

[michaelkruglos]

Example code to reproduce, before this fix is applied:

1.9.3-p484 :002 > Yajl::Parser.parse('["\\ud800\\\\a"]')
irb: yajl_encode.c:192: yajl_string_decode: Assertion `"this should never happen" == ((void *)0)' failed.
Aborted

1.9.3-p484 :001 > require 'yajl'
 => true 
1.9.3-p484 :002 >       Yajl::Parser.parse('["\\ud800\\\\u"]')
irb: yajl_encode.c:108: hexToDigit: Assertion `!(c & 0xF0)' failed.
Aborted

If a specifically crafted JSON is sent to a server, which uses yajl-ruby, it can cause the server to crash, thus potentially creating a DOS attack vector.

[michaelkruglos]

The failures on Travis CI, seem to be related to some environment problem there.

[brianmario]

Closing this in favor of the simpler approach in #178 - apologies for not getting to this sooner!