Undefined-shift in de265_image::set_nonzero_coefficient

[bobfriesenhahn]

Oss-fuzz testing of GraphicsMagick has resulted in oss-fuzz report 384182501. The set_nonzero_coefficient() inline function fails to assure that 'log2TrafoSize - tu_info.log2unitSize' is a positive value. For the provided test-case, the left shift request is -1. The computed value 'width' is used as one of the controlling values in two loops, so this may result in a security issue if the loops run for a very long time, or if the computed 'tu_info' array index is out of bounds. Since the result is "undefined behavior", the behavior may depend on the compiler and compilation options used.

This is the POC test-case that oss-fuzz produced (extension changed to allow upload):