output buffer, don't insert them at all. This prevents a buffer
*underrun* when the substitution consists completely of newlines
(e.g. `echo`) and the byte before the source buffer to which p
points is a '\n', in which case more characters would be removed
from the output buffer than were inserted.
This fixes certain port builds on sparc64.
Approved by: re (scottl)
Reviewed by: des, tjr
99d6402b40