#2129 Metapost lexer may crash with 'interface=none'

[Neil Hodgson]

If an interface declaration comment with 'interface=none' is provided, an out-of-bounds read occurs and there may be a crash.
Example file:

% interface=none
transform pagecoords;
bye

Affected version: 4.2.0 and earlier. Potential fix attached.

[Neil Hodgson]

Fix committed as [88e3b3].