Fix panic on nil map in Fragmentation.

A rare bug occurs when assigning a value to the reassemblers map causes a panic
due to the map being nil. The only way this is possible is if
Fragmentation.Release() is called before Process() is able to finish.

PiperOrigin-RevId: 466771159

pkg/tcpip/network/internal/fragmentation/fragmentation.go

@@ -175,6 +175,10 @@ func (f *Fragmentation) Process(
 	}
 
 	f.mu.Lock()
+	if f.reassemblers == nil {
+		return nil, 0, false, fmt.Errorf("Release() called before fragmentation processing could finish")
+	}
+
 	r, ok := f.reassemblers[id]
 	if !ok {
 		r = newReassembler(id, f.clock)