Fix more bugs that occur when enabling ASan (#293)

src/cmd/ksh93/{bltins/typeset,sh/name,sh/nvtree,sh/nvtype}.c:
- Replace more instances of memcmp with strncmp to fix
  heap-buffer-overflow errors when running the regression tests
  with ASan enabled.

src/cmd/ksh93/edit/vi.c:
- Fix an invalid dereference of the 'p' pointer to fix a crash in
  vi mode when entering a comment in the command history. This
  bugfix was backported from ksh2020:
  https://github.com/att/ast/issues/798

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for the vi mode crash.

src/cmd/ksh93/sh/name.c

@@ -187,7 +187,7 @@ Namval_t *nv_addnode(Namval_t* np, int remove)
 		nv_delete(np,root,NV_NOFREE);
 		np = nv_search(sp->rp->nvname,root,NV_ADD);
 	}
-	if(sp->numnodes && memcmp(np->nvname,NV_CLASS,sizeof(NV_CLASS)-1))
+	if(sp->numnodes && strncmp(np->nvname,NV_CLASS,sizeof(NV_CLASS)-1))
 	{
 		name = (sp->nodes[0])->nvname;
 		i = strlen(name);

src/cmd/ksh93/sh/nvtree.c

@@ -181,7 +181,7 @@ void *nv_diropen(Namval_t *np,const char *name)
 		{
 			char *cp = nv_name(dp->hp);
 			c = strlen(cp);
-			if(memcmp(name,cp,c) || name[c]!='[')
+			if(strncmp(name,cp,c) || name[c]!='[')
 				dp->hp = (Namval_t*)dtnext(dp->root,dp->hp);
 			else
 			{

src/cmd/ksh93/sh/nvtype.c

@@ -883,7 +883,7 @@ Namval_t *nv_mktype(Namval_t **nodes, int numnodes)
 				continue;
 		}
 		nnodes++;
-		if(name && memcmp(&name[m],&np->nvname[m],n)==0 && np->nvname[m+n]=='.')
+		if(name && strncmp(&name[m],&np->nvname[m],n)==0 && np->nvname[m+n]=='.')
 			offset -= sizeof(char*);
 		dsize = nv_datasize(np,&offset);
 		if(!nv_isarray(np) && (dp=(Namtype_t*)nv_hasdisc(np, &type_disc)))