Commit 4b1deb57 added the trim_incomplete_mbyte() call, but at
that time, the function was using fgets() into a fixed-sized
buffer.  The function was passing in "blen - 1" to the size
parameter of fgets, so the check for blen-2 was to see if the
buffer was completely filled by the fgets.

Commit d39d9c0c converted to use a dynamic buffer, but the
trim_incomplete_mbyte() was left in.  It now serves no purpose
because the entire line will be read in.  Presumably the buggy
regexp lib should have been fixed by now too.