Fix always true condition in assert in MacroExpander.cpp.

Fix always true condition in assert in MacroExpander.cpp.

context->index is a size_t, so it's always >= 0.
Let's check that it's > 0 before we decrement it.

Issue: http://code.google.com/p/angleproject/issues/detail?id=349

[mvujovic, 2012-07-18 17:08:06]

Hi Alok,

Could you look at this when you get a chance?

Thanks,
Max

[Alok Priyadarshi, 2012-07-18 17:17:57]

lgtm. Thanks.

[mvujovic, 2012-07-18 17:31:43]

On 2012/07/18 17:17:57, Alok Priyadarshi wrote:
> lgtm. Thanks.

Thanks, Alok. Committed in r1228.