Fix crashes on nullptr

lib/checkclass.cpp

@@ -697,7 +697,8 @@ void CheckClass::assignAllVarsVisibleFromScope(std::vector<Usage>& usageList, co
     for (const Type::BaseInfo& i : scope->definedType->derivedFrom) {
         const Type *derivedFrom = i.type;
 
-        assignAllVarsVisibleFromScope(usageList, derivedFrom->classScope);
+        if (derivedFrom)
+            assignAllVarsVisibleFromScope(usageList, derivedFrom->classScope);
     }
 }
 

lib/tokenize.cpp

@@ -7634,6 +7634,8 @@ void Tokenizer::findGarbageCode() const
                     syntaxError(tok2, "Unexpected token '" + tok2->str() + "'");
             }
         }
+        if (Token::Match(tok, "enum : %num%| {"))
+            syntaxError(tok->tokAt(2), "Unexpected token '" + tok->strAt(2) + "'");
     }
 
     // Keywords in global scope

lib/tokenlist.cpp

@@ -656,7 +656,7 @@ static bool iscpp11init_impl(const Token * const tok)
         if (!Token::Match(colonTok->tokAt(-1), "%name%|%num% :"))
             return false;
         const Token* caseTok = colonTok->tokAt(-2);
-        while (Token::Match(caseTok->tokAt(-1), "::|%name%"))
+        while (caseTok && Token::Match(caseTok->tokAt(-1), "::|%name%"))
             caseTok = caseTok->tokAt(-1);
         return Token::simpleMatch(caseTok, "case");
     };

test/testconstructors.cpp

@@ -1440,6 +1440,14 @@ class TestConstructors : public TestFixture {
                       "[test.cpp:9]: (warning) Member variable 'B::ca' is not assigned a value in 'B::operator='.\n",
                       errout.str());
 
+        check("class C : B {\n"
+              "    virtual C& operator=(C& c);\n"
+              "};\n"
+              "class D : public C {\n"
+              "    virtual C& operator=(C& c) { return C::operator=(c); };\n"
+              "};\n");
+        ASSERT_EQUALS("", errout.str());
+
     }
 
     void initvar_derived_pod_struct_with_union() {

test/testtokenize.cpp

@@ -6759,6 +6759,9 @@ class TestTokenizer : public TestFixture {
 
         // #9445 - typeof is not a keyword in C
         ASSERT_NO_THROW(tokenizeAndStringify("void foo() { char *typeof, *value; }", false, Settings::Native, "test.c"));
+
+        ASSERT_THROW_EQUALS(tokenizeAndStringify("enum : { };"), InternalError, "syntax error: Unexpected token '{'");
+        ASSERT_THROW_EQUALS(tokenizeAndStringify("enum : 3 { };"), InternalError, "syntax error: Unexpected token '3'");
     }