removed FlexArray pattern from CCtxPool

within ZSTDMT_.
This pattern is flagged by less forgiving variants of ubsan
notably used during compilation of the Linux Kernel.

There are 2 other places in the code where this pattern is used.
This fixes just one of them.

lib/compress/zstd_compress.c

@@ -178,6 +178,7 @@ static void ZSTD_freeCCtxContent(ZSTD_CCtx* cctx)
 
 size_t ZSTD_freeCCtx(ZSTD_CCtx* cctx)
 {
+    DEBUGLOG(3, "ZSTD_freeCCtx (address: %p)", (void*)cctx);
     if (cctx==NULL) return 0;   /* support free on NULL */
     RETURN_ERROR_IF(cctx->staticSize, memory_allocation,
                     "not compatible with static CCtx");