Describe the bug

In the ZBUFFv06_createDCtx function in lib/legacy/zstd_v06.c, there is an unchecked allocation failure that can result in a NULL-ptr dereference when calling ZSTD_decompressStream in low-memory conditions. Specifically the following line in ZBUFFv06_createDCtx is unchecked:

zbd->zd = ZSTDv06_createDCtx();

If this allocation fails, a NULL-ptr deference in ZSTDv06_decompressBegin (via ZSTDv06_decompressBegin_usingDict) can be observed.

It looks like there are similar cases in lib/legacy/zstd_v05.c and lib/legacy/zstd_v04.c