FFI/x64: Fix struct-by-value calling conventions.

src/lj_ccall.c

@@ -373,7 +373,7 @@ static int ccall_struct_arg(CCallState *cc, CTState *cts, CType *d, int *rcl,
   dp[0] = dp[1] = 0;
   /* Convert to temp. struct. */
   lj_cconv_ct_tv(cts, d, (uint8_t *)dp, o, CCF_ARG(narg));
-  if (!ccall_struct_reg(cc, dp, rcl)) {  /* Register overflow? Pass on stack. */
+  if (ccall_struct_reg(cc, dp, rcl)) {  /* Register overflow? Pass on stack. */
     MSize nsp = cc->nsp, n = rcl[1] ? 2 : 1;
     if (nsp + n > CCALL_MAXSTACK) return 1;  /* Too many arguments. */
     cc->nsp = nsp + n;