Fix: bug in GetTimeStampFromDER causing crash

[KrisVandermotten]

Resolved / Related Issues

• Closes Bug: Opening the "Signatures" tab causes the application to crash #18510

Steps used to test these changes

1. Downloaded https://github.com/git-for-windows/git/releases/download/v2.54.0.windows.1/Git-2.54.0-64-bit.exe
2. Opened Files and viewed the signatures tab in the properties window for the file downloaded above.
3. Also tested that the method continues to work correctly in case milliseconds are present, e.g. on VisualStudioSetup.exe

@ferrariofilippo