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popen: fix memcpy(dst, NULL, 0)

The `popen_new()` function may be called with inherit_fds = NULL,
nr_inherit_fds = 0. The lua/popen.c code actually does this.

It leads to the following memcpy() call.

```
memcpy(dst, NULL, 0);
```

According to the C11 standard (n1256, 7.21.1), the pointer argument
should have a valid value, which means pointing to some area in the
program's address space, not NULL. The standard doesn't make an
exception for a zero size array.

Personally I doubt that any memcpy() implementation attempts to
dereference the source pointer in case of a zero size, but it is my
assumption, while the standard is the standard.

The problem is found by Coverity.

Follows up #8926

NO_DOC=it is a bug
NO_CHANGELOG=this code is not released yet
NO_TEST=verified by existing test cases, which call popen.new() without
        the inherit_fds option

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