tcg: Return NULL temp for TCG_CALL_DUMMY_ARG

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

tcg/tcg.h

@@ -731,7 +731,7 @@ extern bool parallel_cpus;
 
 static inline TCGTemp *arg_temp(TCGArg a)
 {
-    return &tcg_ctx.temps[a];
+    return a == TCG_CALL_DUMMY_ARG ? NULL : &tcg_ctx.temps[a];
 }
 
 static inline void tcg_set_insn_param(int op_idx, int arg, TCGArg v)