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/bltins/typeset.c

@@ -1581,7 +1581,7 @@ static void print_scan(Sfio_t *file, int flag, Dt_t *root, int option,struct tda
 			if(name)
 			{
 				char *newname = nv_name(np);
-				if(memcmp(name,newname,len)==0 && newname[len]== '.')
+				if(strncmp(name,newname,len)==0 && newname[len]== '.')
 					continue;
 				name = 0;
 			}