Correct "should we log parameters" check

Fixes a regression when the parameter logging was refactored recently.
We can't use the standard way to check at this point, because we're
doing it during invocation, and so haven't set everything up yet. All
we really need to check is that there's a log to write into.

src/spesh/log.c

@@ -123,7 +123,7 @@ void MVM_spesh_log_entry(MVMThreadContext *tc, MVMint32 cid, MVMStaticFrame *sf,
             MVMuint32 i;
             MVMuint32 arg_idx = 0;
             for (i = 0; i < cs->flag_count; i++) {
-                if (!MVM_spesh_log_is_logging(tc))
+                if (!tc->spesh_log)
                     break;
                 if (cs->arg_flags[i] & MVM_CALLSITE_ARG_NAMED)
                     arg_idx++;