Fix datagram buffer remaining size

In the Java version, a datagram was incorrectly rejected if its size was
exactly the remaining buffer size (this was harmless).

In the Rust version, the datagram buffer considered it had enough space
if a packet was 1 byte too big (causing packet overwriting).

Refs #345 <#345>

relay-java/src/main/java/com/genymobile/gnirehtet/relay/DatagramBuffer.java

@@ -65,8 +65,8 @@ public boolean hasEnoughSpaceFor(int datagramLength) {
             // there is at least the extra space for storing 1 packet
             return true;
         }
-        int remaining = tail - head - 1;
-        return HEADER_LENGTH + datagramLength < remaining;
+        int remaining = tail - head - 1; // 1 extra byte to distinguish empty vs full
+        return HEADER_LENGTH + datagramLength <= remaining;
     }
 
     public int capacity() {

relay-rust/src/relay/datagram_buffer.rs

@@ -63,8 +63,9 @@ impl DatagramBuffer {
             // there is at least the extra space for storing 1 packet
             return true;
         }
-        let remaining = self.tail - self.head + 1;
-        HEADER_LENGTH + datagram_length < remaining
+        // 1 extra byte to distinguish empty vs full
+        let remaining = self.tail - self.head - 1;
+        HEADER_LENGTH + datagram_length <= remaining
     }
 
     pub fn write_to<S: DatagramSender>(&mut self, destination: &mut S) -> io::Result<()> {