fixed #4195 segmentation fault of cppcheck (invalid code); Added a testcase

lib/tokenize.cpp

@@ -7282,6 +7282,9 @@ void Tokenizer::simplifyEnum()
                 bool simplify = false;
                 bool hasClass = false;
                 EnumValue *ev = NULL;
+
+                if (!tok1)
+                    return;
                 for (Token *tok2 = tok1->next(); tok2; tok2 = tok2->next()) {
                     if (tok2->str() == "}") {
                         --level;

test/testtokenize.cpp

@@ -56,6 +56,7 @@ private:
         TEST_CASE(tokenize21);  // tokenize 0x0E-7
         TEST_CASE(tokenize22);  // special marker $ from preprocessor
         TEST_CASE(tokenize23);  // tokenize "return - __LINE__;"
+        TEST_CASE(tokenize24);  // #4195 (segmentation fault)
 
         // don't freak out when the syntax is wrong
         TEST_CASE(wrong_syntax1);
@@ -641,6 +642,11 @@ private:
         ASSERT_EQUALS("return -1 ;", tokenizeAndStringify("return - __LINE__;"));
     }
 
+    // #4195 - segfault for "enum { int f ( ) { return = } r = f ( ) ; }"
+    void tokenize24() {
+        tokenizeAndStringify("enum { int f ( ) { return = } r = f ( ) ; }");
+    }
+
     void wrong_syntax1() {
         {
             const std::string code("TR(kvmpio, PROTO(int rw), ARGS(rw), TP_(aa->rw;))");