I used `amount_msat_eq(x, AMOUNT_MSAT(0))` because I forgot this function existed. I probably missed it because the name is surprising, so add "is" in there to make it clear it's a boolean function. You'll note almost all the places which did use it are Eduardo's and Lisa's code, so maybe it's just me. Fix up a few places which I could use it, too. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>