fix error when matching against undefined types

spec/lang/subtyping/interface_spec.lua

@@ -106,6 +106,19 @@ describe("subtyping of interfaces:", function()
       print(foo:get_value())
    ]]))
 
+   it("regression test when matching against an unknown type", util.check_type_error([[
+      local interface B
+      end
+
+      local x: B
+
+      if x is W then
+      end
+   ]], {
+      { msg = "x (of type B) can never be a W" },
+      { msg = "unknown type W" },
+   }))
+
    it("regression test for #830", util.check_lines([[
       local interface IFoo
       end

tl.lua

@@ -8830,8 +8830,10 @@ do
    end
 
    local function a_is_interface_b(self, a, b)
-      assert(a.found)
-      assert(b.found)
+      if (not a.found) or (not b.found) then
+         return false
+      end
+
       local af = a.found.def
       if af.typename == "generic" then
          af = self:apply_generic(a, af, a.typevals)

tl.tl

@@ -8830,8 +8830,10 @@ do
    end
 
    local function a_is_interface_b(self: TypeChecker, a: NominalType, b: NominalType): boolean, {Error}
-      assert(a.found)
-      assert(b.found)
+      if (not a.found) or (not b.found) then
+         return false
+      end
+
       local af = a.found.def
       if af is GenericType then
          af = self:apply_generic(a, af, a.typevals)