patch 9.1.0138: too many STRLEN calls when getting a memline

patch 9.1.0138: too many STRLEN calls when getting a memline

Problem:  too many STRLEN calls when getting a memline
Solution: Optimize calls to STRLEN(), add a few functions in memline.c
          that return the byte length instead of relying on STRLEN()
          (John Marriott)

closes: #14052

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>