p2p: fix ubsan implicit conversion error in CSubNet::ToString()

[jonatack]

Resolves https://cirrus-ci.com/task/4787303177519104?logs=ci#L3020 related to #22584.

UndefinedBehaviorSanitizer: implicit-signed-integer-truncation

netaddress.cpp:1190:18: runtime error: implicit conversion
  from type 'int' of value -1 (32-bit, signed)
  to type 'uint8_t' (aka 'unsigned char')
  changed the value to 255 (8-bit, unsigned)

NetmaskBits() returns -1 if the subnet mask is invalid, which can be incompatible with uint8_t cidr and should probably be exited from.

This allows removing the netaddress UBSan suppression (implicit-signed-integer-truncation).

[tryphe]

untested ACK 835cc11

[tryphe]

I think it would be helpful to add a comment on why the conversion/check is taking place (as described in op).

[maflcko]

Needs a rebase to revert #22584

[jonatack]

Rebased, added comment and dropped the suppression.

[GeneFerneau]

Code review ACK 1416a80

[jonatack]

This fix was opened the same day as the suppression, which was merged while the fix was not. It has two ACKs from the first day or two and I suppose if it was going to be merged, it would have been done so by now.

[maflcko]

I think that a fix for this is still needed. Though, I am not too familiar with this code, so I don't feel comfortable to review or merge this myself.

[maflcko]

Sorry for the wrong comment, I already fixed this in commit efd6f90.