tests/checkasm/checkasm: Give macro a body to avoid potential unexpected syntax issues

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tests/checkasm/checkasm.h

@@ -110,7 +110,7 @@ void checkasm_stack_clobber(uint64_t clobber, ...);
         }\
     } while (0)
 #else
-#define bench_new(...)
+#define bench_new(...) while(0)
 #endif
 
 #endif