exif_scan_JPEG_header: Added length check to prevent information leak

Summary:
'exif_process_SOFn' assumes that the JPEG header has at least 6 length. On providing a length < 6, this leads to an out of bounds heap read.

Fixes CVE-2019-11926

Reviewed By: fredemmott

Differential Revision: D16927050

fbshipit-source-id: 76c4b7c95acbb7852b0435298613d559d94e7270