The current code does not have a bug, but is implicitly casting -1 to
65535 and the sanitizer has no way to know whether we intend that or
not.
```
FUZZ=bitset src/test/fuzz/fuzz /tmp/fuz
error: implicit conversion from type 'int' of value -1 (32-bit, signed)
to type 'value_type' (aka 'unsigned short') changed the value to 65535
(16-bit, unsigned)
Base64: Qv7bX/8=
```
Github-Pull: bitcoin/bitcoin#31431
Rebased-From: edb41e4814