hyperscan: fix minor coverity warning 1358023

hyperscan: fix minor coverity warning 1358023

*** CID 1358023:  Null pointer dereferences  (REVERSE_INULL)
/src/util-mpm-hs.c: 860 in SCHSDestroyThreadCtx()
854         if (thr_ctx->scratch != NULL) {
855             hs_free_scratch(thr_ctx->scratch);
856             mpm_thread_ctx->memory_cnt--;
857             mpm_thread_ctx->memory_size -= thr_ctx->scratch_size;
858         }
859
>>>     CID 1358023:  Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "mpm_thread_ctx->ctx" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
860         if (mpm_thread_ctx->ctx != NULL) {
861             SCFree(mpm_thread_ctx->ctx);
862             mpm_thread_ctx->ctx = NULL;
863             mpm_thread_ctx->memory_cnt--;
864             mpm_thread_ctx->memory_size -= sizeof(SCHSThreadCtx);
865         }