fixed DateComparator if file does not exist

Description:
When a file is deleted after the iterator is created, the accept function throws the following exception: SplFileInfo::getMTime(): stat failed.  This is because the function doesn't check first for the existence of the file.  In theory, a deletion between existence being checked and getMTime getting called would still result in this error, but the risk area for this race condition is much smaller than the current risk area.

| Q             | A
| ------------ | ----
| Bug fix? | yes
| New feature? | no
| BC breaks? | no
| Deprecations? | no
| Tests pass? | yes
| Fixed tickets | #11785
| License | MIT
| Doc PR | 

I just ran a benchmark for curiosity.  On my Intel Haswell i3 system with php running in an ubuntu VM within Windows 8, 10000 calls to file_exists took 0.01 seconds regardless of whether or not the file actually existed.  So I'd consider the addition of this check to be negligible.