[YAMLIO] Correctly diagnose empty alias/anchor

[scott.linder]

Summary

The Range of an alias/anchor token includes the leading & or *,
but it is skipped while parsing the name. The check for an empty name
fails to account for the skipped leading character and so the error is
never hit.

Fix the off-by-one and add a couple regression tests.

[dexonsmith]

LGTM, thanks!

llvm/lib/Support/YAMLParser.cpp

if (   *Current == '[' || *Current == ']'
       break;
     StringRef::iterator i = skip_ns_char(Current);
     if (i == Current)
       break;
     Current = i;
     ++Column;
   }
 
-  if (Start == Current) {
+  if (Start + 1 == Current) {
     setError("Got empty alias or anchor", Start);
     return false;
   }
 
   Token T;
   T.Kind = IsAlias ? Token::TK_Alias : Token::TK_Anchor;
   T.Range = StringRef(Start, Current - Start);
   TokenQueue.push_back(T);

llvm/unittests/Support/YAMLIOTest.cpp

TEST(YAMLIO, TestUnknownDirective) {
   Input yin("%");
   EXPECT_FALSE(yin.setCurrentDocument());
   EXPECT_TRUE(yin.error());
 
   Input yin2("%)");
   EXPECT_FALSE(yin2.setCurrentDocument());
   EXPECT_TRUE(yin2.error());
 }
+
+TEST(YAMLIO, TestEmptyAlias) {
+  Input yin("&");
+  EXPECT_FALSE(yin.setCurrentDocument());
+  EXPECT_TRUE(yin.error());
+}
+
+TEST(YAMLIO, TestEmptyAnchor) {
+  Input yin("*");
+  EXPECT_FALSE(yin.setCurrentDocument());
+  EXPECT_TRUE(yin.error());
+}