C++: add good example for DeadCodeGoto

Author: Robert Marsh

cpp/ql/src/Critical/DeadCodeGoto.cpp

@@ -1,3 +1,7 @@
-goto err;
-free(pointer);
-err: return -1;
+goto err1;
+free(pointer); // BAD: this line is unreachable
+err1: return -1;
+
+free(pointer); // GOOD: this line is reachable
+goto err2;
+err2: return -1;