🚀 go-pugleaf

Cu seems to keep track of what the user types in a line buffer.
There is no check for overflowing this buffer.  Since one can easily
go for a long time in a screen editor without getting a return, it is easy
to core dump the receive process.  The following patch seems to fix it:
at the beginning of the routine rd, before *p++=c; add
	if (p >= &b[sizeof b]) p = b;
This fix was arrived at quickly, and it is possible that the author will
prefer a different fix, such as
	if (p >= &b[sizeof b]) p--;
after *p++=c;.  Actually, it seems like cu has little business keeping
a line buffer unless the first character was a tilde.