editors/nvi2: Fix core dump when tags file pattern has a trailing '\'

If you create a tags file of a macro that ends with a '\' and tag
for it, vi dumps core. For example:

    zinc 76 % cat test.h
    #define LATIN2PLAIN(ch) (((u_char)ch) >= 0x80 ? \
       pgm_read_byte_far(pgm_get_far_address(latin2plain) + \
       (((u_char)ch) - 0x80)) : (isprint(ch) ? (ch) : '_'))
    zinc 77 % ctags test.h
    zinc 78 % vi -t LATIN2PLAIN
    Segmentation fault

The problem is that the loop variable is unsigned (size_t) and it
gets decremented twice: 1 -> 0 -> 4294967295

Apply the upstream patch to solve this:

    https://github.com/lichray/nvi2/pull/111

editors/nvi2/Makefile

@@ -1,6 +1,7 @@
 PORTNAME=	nvi2
 PORTVERSION=	2.2.0
 DISTVERSIONPREFIX=	v
+PORTREVISION=	1
 CATEGORIES=	editors
 
 MAINTAINER=	leres@FreeBSD.org

editors/nvi2/files/patch-ex_ex__subst.c

@@ -0,0 +1,12 @@
+--- ex/ex_subst.c.orig	2020-08-01 22:27:51 UTC
++++ ex/ex_subst.c
+@@ -1194,7 +1194,8 @@ re_tag_conv(SCR *sp, CHAR_T **ptrnp, size_t *plenp, in
+ 	for (; len > 0; --len) {
+ 		if (p[0] == '\\' && (p[1] == '/' || p[1] == '?')) {
+ 			++p;
+-			--len;
++			if (len > 1)
++				--len;
+ 		} else if (STRCHR(L("^.[]$*"), p[0]))
+ 			*t++ = '\\';
+ 		*t++ = *p++;