[#3256] attempt to fix nonsense Wstringop-overread

[#3256] attempt to fix nonsense Wstringop-overread

```
warning: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’ reading 1 or more bytes from a region of size 0 [-Wstringop-overread]
```

and Wsign-compare in the same file:

```
test_control.cc:116:35: warning: comparison of integers of different signs: 'uint64_t' (aka 'unsigned long') and 'int' [-Wsign-compare]                                                                                                                         338ms
test_control.cc:491:43: warning: comparison of integers of different signs: 'size_type' (aka 'unsigned long') and 'const int' [-Wsign-compare]
test_control.cc:529:50: warning: comparison of integers of different signs: 'size_type' (aka 'unsigned long') and 'const int' [-Wsign-compare]
```