Dereference of null pointer #5353 (#5376)

add an assert to make sure that `key_field` is not a null pointer.

src/idl_gen_general.cpp

@@ -1450,6 +1450,7 @@ class GeneralGenerator : public BaseGenerator {
     // Only generate key compare function for table,
     // because `key_field` is not set for struct
     if (struct_def.has_key && !struct_def.fixed) {
+      FLATBUFFERS_ASSERT(key_field);
       if (lang_.language == IDLOptions::kJava) {
         code += "\n  @Override\n  protected int keysCompare(";
         code += "Integer o1, Integer o2, ByteBuffer _bb) {";