kernel_2d_fixed giving false assertion errors

### Actual behavior

``` boost::gil::detail::kernel_2d_fixed ``` gives false assertion errors during its initialization.

### Expected behavior
A 2D kernel with provided values and center co-ordinates should be created

### C++ Minimal Working Example
https://wandbox.org/permlink/apUAhyheZUyea3UO

```cpp
#include
namespace gil = boost::gil;
int main
{
// your code
std::array v = {0};
gil::detail::kernel_2d_fixed kernel(v.cbegin(), 4, 4); // Not ok
}
```
This problem remained unnoticed in CI build due to [this](https://github.com/boostorg/gil/blob/7dd61209a81695600d773f481bb8b22b0f74a30a/test/extension/numeric/kernel_fixed.cpp#L9) macro.
### Environment

- Compiler version: gcc 11.1.0
- Version: Boost 1.76.0