Fix #13694 fuzzing crash (null-pointer-use) in isUnreachableOperand()

Author: web-flow

lib/tokenize.cpp

@@ -8944,6 +8944,8 @@ void Tokenizer::findGarbageCode() const
             syntaxError(tok);
         if (Token::Match(tok, "typedef [,;:]"))
             syntaxError(tok);
+        if (Token::Match(tok, "? %assign%"))
+            syntaxError(tok);
         if (Token::Match(tok, "!|~ %comp%") &&
             !(cpp && tok->strAt(1) == ">" && Token::simpleMatch(tok->tokAt(-1), "operator")))
             syntaxError(tok);

test/cli/fuzz-crash/crash-a03d001b1336e191debffcde652ce5fb63d0a7d6

@@ -0,0 +1 @@
+(c[?=3:4])p