message: [idl_parser, JSON] Disable parsing of JSON for incomplete schemes
  This commit disable JSON parsing for an incomplete scheme if JSON object is embedded into one file with the scheme.
  This should improve the quality of OSS-Fuzz inputs for the parser_fuzzer target.
diff: @@ -3189,6 +3189,9 @@ CheckedError Parser::ParseRoot(const char *source, const char **include_paths,
  }
  }
  }
  + // Parse JSON object only if the scheme has been parsed.
  + if (token_ == '{') { ECHECK(DoParseJson()); }
  + EXPECT(kTokenEof);
  return NoError();
  }
  @@ -3308,7 +3311,7 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
  } else if (IsIdent("namespace")) {
  ECHECK(ParseNamespace());
  } else if (token_ == '{') {
  - ECHECK(DoParseJson());
  + return NoError();
  } else if (IsIdent("enum")) {
  ECHECK(ParseEnum(false, nullptr));
  } else if (IsIdent("union")) {
  