Use saturating addition for ephemeral timers

Integer overflows crash the application by default.

On a first sight this is only a potential crash that can't be
triggered, because timestamps are stored as i64 and ephemeral timer
duration is u32.

src/dc_receive_imf.rs

@@ -1046,7 +1046,9 @@ INSERT INTO msgs
         } else {
             match ephemeral_timer {
                 EphemeralTimer::Disabled => 0,
-                EphemeralTimer::Enabled { duration } => rcvd_timestamp + i64::from(duration),
+                EphemeralTimer::Enabled { duration } => {
+                    rcvd_timestamp.saturating_add(duration.into())
+                }
             }
         };