Fix crash during ^W. From OpenBSD.

Original commit message by millert@openbsd:
When ^W (WERASE) is hit in insert mode it's possible that the line
buffer is accessed out of bounds. If 'max' == 0 and 'tp->cno' == 1
the 'tp->cno' value is first reduced by one and then 'tp->lb' is
accessed at 'tp->cno' - 1.  Also remove dead (and incorrect) code
in the TXT_ALTWERASE case.  From Arto Jonsson; OK martynas@