Fix: avoid crashes on non-breaking spaces in the text.

[bjones1]

This fixes a crash when a non-breaking space is in the input text.

Looking at the implementation of is_list_word, it will always exit when it encounters whitespace; the debug assertion isn't necessary. A look at the function shows that it implements a state machine:

stateDiagram-v2
    false: return false
    true: return true
    [*] --> Numeric: is_numeric
    [*] --> false: !is_numeric | end of string
    Numeric --> Numeric: is_numeric
    Numeric --> false: !is_numeric | end of string
    Numeric --> Delim: "." | ")"
    Delim --> false: !end of string
    Delim --> true: end of string