segfault in canvas.streamJPEGSync

[stroncium]

Calling streamJPEGSync with bufSize = 0 sometimes causes Segmentation Fault.

Cairo version: 1.12.12
JPEG version: 8
(reported by node-canvas)
nodejs version: v0.11.3-pre (git head one week old or so)

I've also encountered cases when it was throwing something like "corrupted double-linked list" error, but I can't reproduce it now and logs are out of history.

[zbjornson]

@Hakerh400 says they can't repro in #1245 (comment), so closing.