Fix off-by-one error when trying to remove delimiter from section name

A MOOSE user reported that they couldn't create section names in a getpot file like this 

`  [./Somename/a]`

While the second slash is not prohibited in the example above, the logic in `GetPot::_process_section_name` incorrectly subtracts from the length of the string when doing an "in-bounds" comparison creating an infinite loop and chewing up memory.