common/amount: fix OpenBSD compile warning.

```
cc common/amount.c
common/amount.c:306:15: error: implicit conversion from 'unsigned long long' to
      'double' changes value from 18446744073709551615 to 18446744073709551616
      [-Werror,-Wimplicit-int-float-conversion]
        if (scaled > UINT64_MAX)
                   ~ ^~~~~~~~~~
/usr/include/sys/stdint.h:123:21: note: expanded from macro 'UINT64_MAX'
                                ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
gmake: *** [Makefile:254: common/amount.o] Error 1
bsd$
```

Fixes: #4044
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/amount.c

@@ -303,7 +303,9 @@ WARN_UNUSED_RESULT bool amount_msat_scale(struct amount_msat *val,
 {
 	double scaled = sat.millisatoshis * scale;
 
-	if (scaled > UINT64_MAX)
+	/* If mantissa is < 64 bits, a naive "if (scaled >
+	 * UINT64_MAX)" doesn't work.  Stick to powers of 2. */
+	if (scaled >= (double)((u64)1 << 63) * 2)
 		return false;
 	val->millisatoshis = scaled;
 	return true;