Eliminate Undefined variable errors in wpsc_obtain_the_description

This poorly-written function is now less poorly-written. Removed
unneeded globals, test $_GET variables before using, removed direct
query to database
Typo in last commit

Got too carried away
Stop testing for category ID - just return output if set

Poor logic begets poor logic
Merge pull request #288 from 1bigidea/undefined_cat_id

Eliminate Undefined variable errors in wpsc_obtain_the_description