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/edit/vi.c

@@ -992,7 +992,7 @@ static int cntlmode(Vi_t *vp)
 				c = (virtual[0]=='#');
 				while(p-- >= virtual)
 				{
-					if(*p=='\n' || p<virtual)
+					if(p<virtual || *p=='\n')
 					{
 						if(c) /* delete '#' */
 						{