Handle realpath()/getRealPath() failures

[bpfoley]

Found by phpstan analyze --level 7

[cpeel]

If $real_path is false here the file doesn't exist (which I'm unclear on how that can actually be possible given that RecursiveIteratorIterator just told us about it, but let's humor PHPstan's paranoia about race conditions) and it would be clearer to continue than try to do something with the file / directory.

Changes like this seem like we're trading readability for PHPstan's paranoia.

[bpfoley]

PHPStan knows nothing about race conditions or the inner workings of PHP's realpath, all it knows is the function signature in one of its stub files. Unfortunately, the underlying libc function realpath() can fail for a variety of (admittedly unlikely) reasons even if the file/directory does exist. PHP returns false in those cases. See https://man7.org/linux/man-pages/man3/realpath.3.html#ERRORS

I'm not sure silently suppressing the error with if ($real_path === false) continue; is the right thing to do, especially if the reason is that something else on the system has caused the directory to be inaccessible (eg a sysadmin doing chmod 0 on the uploads directory). In that case realpath would silently fail multiple times potentially leaving an unflattened directory behind it. But given the aim is to get PHPstan level 7 working so we can detect more useful missing error handling, I don't really mind too much what way we handle this.