Fix an undefined variable dereference

[msm-code]

This fixes the following error in the webide when parsing some JPEG files:

Parse error: undefined
Call stack: undefined TypeError: ksyType.instancesByJsName is undefined

This issue was confirmed by:

• me
• two of my friends (every one of us trying a different JPEG file)
• people in a 2 yers old discussion thread: https://gitter.im/kaitai_struct/Lobby?at=5d6e5bb238499c13a69268b8

Before:

After:

[msm-code]

File for testing. Confirmed not working (before the fix) by two independent friends, on chrome and on firefox. Uploaded as zip to prevent reencoding by github (just in case), please extract before testing.

testcase.zip