FoundationEssentials: drop invalid modification dates (#628)

Follow the expected behaviour to drop invalid modification dates, e.g.
`+NaN`. This matches the semantics that are expected on Unix and avoids
a crash due to the initialiser.

Author: compnerd

Sources/FoundationEssentials/FileManager/FileManager+Files.swift

@@ -718,7 +718,10 @@ extension _FileManagerImpl {
                 let seconds = modification.timeIntervalSince1601
 
                 var uiTime: ULARGE_INTEGER = .init()
-                uiTime.QuadPart = UInt64(seconds * 10000000)
+                guard let converted = UInt64(exactly: seconds * 10000000.0) else {
+                    return
+                }
+                uiTime.QuadPart = converted
 
                 var ftTime: FILETIME = .init()
                 ftTime.dwLowDateTime = uiTime.LowPart