MIPS64: Fix xpcall() error case.

Thanks to François Perrad and Stefan Pejic.

src/vm_mips64.dasc

@@ -1399,15 +1399,16 @@ static void build_subroutines(BuildCtx *ctx)
   |.  nop
   |
   |.ffunc xpcall
-  |  daddiu NARGS8:RC, NARGS8:RC, -16
+  |  daddiu NARGS8:TMP0, NARGS8:RC, -16
   |  ld CARG1, 0(BASE)
   |   ld CARG2, 8(BASE)
-  |    bltz NARGS8:RC, ->fff_fallback
+  |    bltz NARGS8:TMP0, ->fff_fallback
   |.    lbu TMP1, DISPATCH_GL(hookmask)(DISPATCH)
   |  gettp AT, CARG2
   |  daddiu AT, AT, -LJ_TFUNC
   |  bnez AT, ->fff_fallback		// Traceback must be a function.
   |.   move TMP2, BASE
+  |  move NARGS8:RC, NARGS8:TMP0
   |   daddiu BASE, BASE, 24
   |  // Remember active hook before pcall.
   |  srl TMP3, TMP3, HOOK_ACTIVE_SHIFT