MessagingImageMessage: Don't crash if measured after recycle

We're hoping to gut the recycling code in the messaging notification
code entirely (it's a lot of complexity for apparently little or no
performance gain), but in the meantime, mitigate a System UI crash.

Bug: 231025249
Test: none, I found no unit tests to start with
Change-Id: I3bcc14d102a87ed96ce37241d25eccf44ba2acb0