TurboJPEG: Avoid dangling pointers

TurboJPEG: Avoid dangling pointers

This addresses a minor concern (LJT-01-002) expressed in a security
audit by Cure53.  _tjInitCompress() and _tjInitDecompress() call
(respectively) jpeg_mem_dest_tj() and jpeg_mem_src_tj() with a pointer
to a dummy buffer, in order to set up the destination/source manager.
The dummy buffer should never be used, but it's still better to make it
static so that the pointer in the destination/source manager always
points to a valid region of memory.