wallet: fix reference to potentially-undefined variable.

If all is set, diff may not be:

    ==1230518== Use of uninitialised value of size 8
    ==1230518==    at 0x4C5781B: _itoa_word (_itoa.c:179)
    ==1230518==    by 0x4C736F4: __vfprintf_internal (vfprintf-internal.c:1687)
    ==1230518==    by 0x4C88119: __vsnprintf_internal (vsnprintf.c:114)
    ==1230518==    by 0x1D44B6: do_vfmt (str.c:66)
    ==1230518==    by 0x1D45A0: tal_vfmt_ (str.c:92)
    ==1230518==    by 0x1D4401: tal_fmt_ (str.c:44)
    ==1230518==    by 0x15D30F: fmt_amount_sat (amount.c:60)
    ==1230518==    by 0x15D338: fmt_amount_sat_ (amount.c:62)
    ==1230518==    by 0x178C45: type_to_string_ (type_to_string.c:35)
    ==1230518==    by 0x1B8F75: json_fundpsbt (reservation.c:394)
    ==1230518==    by 0x12D0EC: command_exec (jsonrpc.c:602)
    ==1230518==    by 0x12D598: rpc_command_hook_callback (jsonrpc.c:712)

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

wallet/reservation.c

@@ -324,9 +324,10 @@ static struct command_result *json_fundpsbt(struct command *cmd,
 						   struct amount_sat,
 						   amount),
 				    tal_count(utxos),
-				    type_to_string(tmpctx,
-						   struct amount_sat,
-						   &diff));
+				    all ? "all"
+				    : type_to_string(tmpctx,
+						     struct amount_sat,
+						     &diff));
 	}
 
 	if (all) {