Rather than break the API, use total capacity here.
```
Valgrind error file: valgrind-errors.5880
==5880== Use of uninitialised value of size 8
==5880== at 0x4A390BB: _itoa_word (_itoa.c:183)
==5880== by 0x4A43C9B: __printf_buffer (vfprintf-process-arg.c:155)
==5880== by 0x4A69D90: vsnprintf (vsnprintf.c:96)
==5880== by 0x1875E6: json_out_addv (json_out.c:239)
==5880== by 0x14471E: json_add_primitive_fmt (json_stream.c:170)
==5880== by 0x144BA2: json_add_u64 (json_stream.c:282)
==5880== by 0x145E33: json_add_amount_msat (json_stream.c:619)
==5880== by 0x11DDE2: channel_hint_to_json (channel_hint.c:33)
==5880== by 0x11FE9F: channel_hint_notify_core (libplugin-pay.c:394)
==5880== by 0x11FF7A: channel_hint_notify (libplugin-pay.c:412)
==5880== by 0x1201EA: channel_hints_update (libplugin-pay.c:455)
==5880== by 0x122DAF: handle_intermediate_failure (libplugin-pay.c:1437)
==5880==
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).
03025469be