log: truncate giant IO logging.

Adding a giant IO message simply causes it to be pruned immediately,
so truncate it if it's more than 1/64 the max size.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/test/run-invoice-select-inchan.c

@@ -382,7 +382,7 @@ void set_log_outfn_(struct log_book *lr UNNEEDED,
 				  bool continued UNNEEDED,
 				  const struct timeabs *time UNNEEDED,
 				  const char *str UNNEEDED,
-				  const u8 *io UNNEEDED,
+				  const u8 *io UNNEEDED, size_t io_len UNNEEDED,
 				  void *arg) UNNEEDED,
 		    void *arg UNNEEDED)
 { fprintf(stderr, "set_log_outfn_ called!\n"); abort(); }