Fix serialize_datetime (edits)

Add checks to prevent `struct.pack()` from failing and for users to not use dates outside the ranges of 1970 to 2038.

telethon/tl/tlobject.py

@@ -136,6 +136,10 @@ class TLObject:
         except OSError:
             dt = 0
 
+        # 2145916800 is the date 2038, 1, 1.
+        if dt > 2145916800:
+            raise ValueError("Date is outside the range of valid values.")
+
         if isinstance(dt, int):
             return struct.pack('<i', dt)