Fix #546 fuzzing crash in simplecpp::preprocess() (#553)

Author: chrchr-github

simplecpp.cpp

@@ -3602,9 +3602,11 @@ void simplecpp::preprocess(simplecpp::TokenList &output, const simplecpp::TokenL
                                     header = tok->str().substr(1U, tok->str().size() - 2U);
                                     closingAngularBracket = true;
                                 }
-                                std::ifstream f;
-                                const std::string header2 = openHeader(f,dui,sourcefile,header,systemheader);
-                                expr.push_back(new Token(header2.empty() ? "0" : "1", tok->location));
+                                if (tok) {
+                                    std::ifstream f;
+                                    const std::string header2 = openHeader(f,dui,sourcefile,header,systemheader);
+                                    expr.push_back(new Token(header2.empty() ? "0" : "1", tok->location));
+                                }
                             }
                             if (par)
                                 tok = tok ? tok->next : nullptr;

test.cpp

@@ -3259,6 +3259,12 @@ static void fuzz_crash()
                             "foo(f##oo(intp))\n";
         (void)preprocess(code, simplecpp::DUI()); // do not crash
     }
+    { // #546
+        const char code[] = "#if __has_include<\n";
+        simplecpp::OutputList outputList;
+        ASSERT_EQUALS("", preprocess(code, &outputList)); // do not crash
+        ASSERT_EQUALS("file0,1,syntax_error,failed to evaluate #if condition\n", toString(outputList));
+    }
 }
 
 // memory leaks detected by LSAN/valgrind