Usage of disabled protocol

When a protocol selection parameter option disables all protocols without
adding any then the default set of protocols would remain in the allowed set
due to an error in the logic for removing protocols. The below command would
perform a request to curl.se with a plaintext protocol which has been
explicitly disabled.

curl --proto -all,-http http://curl.se

The flaw is only present if the set of selected protocols disables the entire
set of available protocols, in itself a command with no practical use and
therefore unlikely to be encountered in real situations. The curl security team
has thus assessed this to be low severity bug.