Check that there are no assignments in assertions

[Gumix]

It seems that compiler doesn't produce warnings when = is used instead of == inside assert():

tarantool$ git grep "assert.* = " src/
src/box/lua/net_box.c:3165:        assert(transport->self_ref = LUA_NOREF);

tarantool-ee$ git grep "assert.* = " src/
src/box/arrow.c:903:        assert(array->n_buffers = 3);
src/box/arrow.c:938:        assert(array->n_buffers = 2);
src/box/memcs_engine.c:329:        assert(stmt->new_row.count = space->format->exact_field_count);