lib/model: Don't panic if block size in index is larger than protocol block size

This doesn't happen today, but it might in the future if the block size
were increased or made variable and we were talking to a client from the
future.

lib/model/rwfolder.go

@@ -1260,7 +1260,11 @@ func (f *sendReceiveFolder) copierRoutine(in <-chan copyBlocksState, pullChan ch
 				continue
 			}
 
-			buf = buf[:int(block.Size)]
+			if s := int(block.Size); s > cap(buf) {
+				buf = make([]byte, s)
+			} else {
+				buf = buf[:s]
+			}
 
 			found, err := weakHashFinder.Iterate(block.WeakHash, buf, func(offset int64) bool {
 				if _, err := verifyBuffer(buf, block); err != nil {