Followup fix for #14126 crash in Tokenizer::simplifyCPPAttribute() (#7863)

Author: chrchr-github

lib/tokenize.cpp

@@ -9550,6 +9550,8 @@ void Tokenizer::simplifyCPPAttribute()
                 Token* head = skipCPPOrAlignAttribute(tok)->next();
                 while (isCPPAttribute(head) || isAlignAttribute(head))
                     head = skipCPPOrAlignAttribute(head)->next();
+                if (!head)
+                    syntaxError(tok);
                 head->isAttributeUnused(true);
             } else if (Token::Match(tok->previous(), ") [ [ expects|ensures|assert default|audit|axiom| : %name% <|<=|>|>= %num% ] ]")) {
                 const Token *vartok = tok->tokAt(4);

test/testgarbage.cpp

@@ -1771,6 +1771,7 @@ class TestGarbage : public TestFixture {
     }
     void garbageCode229() { // #14126
         ASSERT_THROW_INTERNAL(checkCode("void f() {} [[maybe_unused]]"), SYNTAX);
+        ASSERT_THROW_INTERNAL(checkCode("void f() {} [[unused]]"), SYNTAX);
     }