null dereference in QuickTimeVideo::userDataDecoder

[kevinbackhouse]

Reported by OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=51751

There is a null dereference here:

exiv2/src/quicktimevideo.cpp

Line 833 in a2cb06a

xmpData_[exvGettext(tv->label_)] = Exiv2::toString(buf.data());

The problem is that tv can be null.

quicktimevideo.cpp is a new file, so this bug doesn't affect any released versions of exiv2.

poc: https://user-images.githubusercontent.com/4358136/194719331-b9106875-7031-427b-8e78-407620bf7ba6.mp4

To reproduce:

exiv2 poc.mp4

(Causes exiv2 to crash.)