[1.2.X] Fixed #13568 -- Fixed the blocktrans tag to not raise a KeyError if...

[1.2.X] Fixed #13568 -- Fixed the blocktrans tag to not raise a KeyError if the number of variables in the singular and the plural block differ. Thanks, deloide.

Backport from trunk (r13967).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13973 bcc190cf-cafb-0310-a4f2-bffc1f526a37