Limits U and O entries size

Fixes document received in https://bugzilla.mozilla.org/show_bug.cgi?id=900822 comment 9

abcpdf adds extra character to the "O" entry of the standard encryption dictionary, which makes it incorrect from PDF32000 specification point of view.

(Workaround for patching files generated by abcpdf: open binary PDF data,  look for binary '2F 4F 20' starting from the end if the file, and make this /O entry one character shorter by searching and replacing binary '0A 29' [located somewhere around 100 bytes after /O] to '29 20')