UB in tcd.c opj_tcd_dc_level_shift_decode - pointer arithmetic on NULL pointer

[pauldreik]

Using ubsan and afl, I found that pointer arithmetic may be carried out on a null pointer, which is undefined behaviour.

See #1442 for code to reproduce it.

[rouault]

fixed per fixed per https://github.com/uclouvain/openjpeg/pull/1496