remove length in ShmSocket ctor, not used.

lichao

2021-06-01 f7f8fbc85c48b003d1e3094a94a3528001977977

 utest/simple_tests.cpp

@@ -50,12 +50,12 @@
         typedef ShmObject<s1000> Int;
         std::string name = std::to_string(id);
         auto a0 = Avail();
         Int i1(shm, name);
         Int i1(shm, name, eOpenOrCreate);
         auto a1 = Avail();
         BOOST_CHECK_LT(a1, a0);
         printf("s1000 size: %ld\n", a0 - a1);
         i1->a[0] = 5;
         Int i2(shm, name);
         Int i2(shm, name, eOpenOrCreate);
         auto a2 = Avail();
         BOOST_CHECK_EQUAL(a1, a2);
         BOOST_CHECK_EQUAL(i1.data(), i2.data());