checksrc: handle zero scoped ignore commands

If a !checksrc! disable command specified to ignore zero errors, it was
still added to the ignore block even though nothing was ignored. While
there were no blocks ignored that shouldn't be ignored, the processing
ended with with a warning:

<filename>:<line>:<col>: warning: Unused ignore: LONGLINE (UNUSEDIGNORE)
 /* !checksrc! disable LONGLINE 0 */
                    ^
Fix by instead treating a zero ignore as a a badcommand and throw a
warning for that one.

Closes #3096
Reviewed-by: Daniel Stenberg <daniel@haxx.se>