gcc-12 (Ubuntu 12.3.0-9ubuntu2) 12.3.0 with -O3 gives:
```
In file included from plugins/renepay/test/../mcf.c:5,
from plugins/renepay/test/run-arc.c:13:
ccan/ccan/tal/str/str.h: In function ‘minflow’:
ccan/ccan/tal/str/str.h:43:9: error: ‘errmsg’ may be used uninitialized [-Werror=maybe-uninitialized]
43 | tal_fmt_(ctx, TAL_LABEL(char, "[]"), __VA_ARGS__)
| ^~~~~~~~
plugins/renepay/test/../mcf.c:1565:15: note: ‘errmsg’ was declared here
1565 | char *errmsg;
| ^~~~~~
cc1: all warnings being treated as errors
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
e3c4bdf209