avoid splitting UTF-8 encoded character in the middle of encoded bytes

The file format spec in RFC 6350 says:
http://tools.ietf.org/html/rfc6350#section-3.2
Line Delimiting and Folding
"Multi-octet characters MUST remain contiguous."
This patch avoids splitting an UTF-8 encoded character in the middle
of the encoded bytes
BUG: 271859
FIXED-IN: 4.13
REVIEW: 116933