bug#10940: Emacs 24: regression reading comma

[Chris Leyon]

In Emacs 23, the lisp reader correctly parses a comma when reading an
event modifier, as in the following example specifying the character
C-,
    (global-set-key [(control ,)] 'my-command)

The Emacs 24 reader gives the following error when parsing the same expression:
    Invalid read syntax: )