Fix theoretical array overflow in lua_typename()

Out of the 12 defects in lua that coverity reports, 5 of them involve
`lua_typename()` and out of the dozens of defects in ZFS that lua
reports, 3 of them involve `lua_typename()` due to the ZCP code. Given
all of the uses of `lua_typename()` in the ZCP code, I was surprised
that there were not more. It appears that only 2 were reported because
only 3 called `lua_type()`, which does a defective sanity check that
allows invalid types to be passed.

lua/lua@d4fb848be7 addressed this in
upstream lua 5.3. Unfortunately, we did not get that fix since we use
lua 5.2 and we do not have assertions enabled in lua, so the upstream
solution would not do anything.

While we could adopt the upstream solution and enable assertions, a
simpler solution is to fix the issue by making `lua_typename()` return
`internal_type_error` whenever it is called with an invalid type. This
avoids the array overflow and if we ever see it appear somewhere, we
will know there is a problem with the lua interpreter.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13947

module/lua/lapi.c

@@ -251,6 +251,8 @@ LUA_API int lua_type (lua_State *L, int idx) {
 
 LUA_API const char *lua_typename (lua_State *L, int t) {
   UNUSED(L);
+  if (t > 8 || t < 0)
+    return "internal_type_error";
   return ttypename(t);
 }