Assign None to code C50

[leovam]

In the line 127 to line 129 in __init__():

sblock, eblock = block.split('-') # A00-B99
salpha, snumeric = ord(sblock[0]), int(sblock[1:].lstrip('0') or 0)
ealpha, enumeric = ord(eblock[0]), int(eblock[1:].lstrip('0') or 0)

This does not work for code C50, since chapter II start from C00 and end with D48, so 50 will larger than 0 but less than 48.

A condition should be added here, thanks

[wbazant]

This will work:

       if salpha == ealpha:
            return alpha == salpha and snumeric <= numeric <= enumeric
        else:
            return (salpha <= alpha < ealpha and snumeric <= numeric) or (salpha < alpha <= ealpha and numeric <= enumeric)

#1 (comment)