[largeNbDicts] Second try at fixing decompression segfault to always create compressI...

[largeNbDicts] Second try at fixing decompression segfault to always create compressInstructions

Summary:
Freeing an uninitialized pointer is undefined behavior. This caused a segfault
when compiling the benchmark with Clang -O3 and benching decompression.

V2: always create compressInstructions but check if cctxParams is NULL before
setting CCtx params to avoid segfault.

Test Plan:
make and run