[patch] fix for infinite recursion in lookup_symbol

[Jim Blandy]

I've committed Jim Ingham's fix.

Folks, I do apologize.  For partly personal reasons, I've been very
much in the ozone regarding almost every GDB-related matter for quite
some time.  I'll try to shape up.

Ideally, I'd be ready to pass judgement on whether the famous October
12 changes should be reverted, but I don't feel like I have a handle
on the situation at the moment.  I'll try to make a real judgement on
the issue tomorrow.  In the meantime, Jim I's patch seems perfectly
safe.