Fix heap-buffer-overflow if there is a struct within a union

The validator previously did not check if a struct within a union was
valid, causing a heap buffer overflow.  Add a check to make sure that
the struct is valid in this case.

Change-Id: I87d41b12fdfc2a99406789531ba92b841c063c76

src/idl_gen_cpp.cpp

@@ -1213,7 +1213,8 @@ class CppGenerator : public BaseGenerator {
             "      auto ptr = reinterpret_cast<const {{TYPE}} *>(obj);";
         if (ev.union_type.base_type == BASE_TYPE_STRUCT) {
           if (ev.union_type.struct_def->fixed) {
-            code_ += "      return true;";
+            code_ += "      return verifier.Verify<{{TYPE}}>(static_cast<const "
+                     "uint8_t *>(obj), 0);";
           } else {
             code_ += getptr;
             code_ += "      return verifier.VerifyTable(ptr);";

tests/union_vector/union_vector_generated.h

@@ -547,13 +547,13 @@ inline bool VerifyCharacter(flatbuffers::Verifier &verifier, const void *obj, Ch
       return verifier.VerifyTable(ptr);
     }
     case Character_Rapunzel: {
-      return true;
+      return verifier.Verify<Rapunzel>(static_cast<const uint8_t *>(obj), 0);
     }
     case Character_Belle: {
-      return true;
+      return verifier.Verify<BookReader>(static_cast<const uint8_t *>(obj), 0);
     }
     case Character_BookFan: {
-      return true;
+      return verifier.Verify<BookReader>(static_cast<const uint8_t *>(obj), 0);
     }
     case Character_Other: {
       auto ptr = reinterpret_cast<const flatbuffers::String *>(obj);