Partners footer bug fix

Checks for existence of `$vars['field_partners']` before preprocessing it.

These errors were occurring viewing the page in question:

````
Warning: Invalid argument supplied for foreach() in dosomething_helpers_preprocess_partners_vars() (line 88 of /var/www/beta.dosomething.org/releases/20141209142557/lib/modules/dosomething/dosomething_helpers/dosomething_helpers.theme.inc).

Warning: Invalid argument supplied for foreach() in dosomething_helpers_format_partners_list() (line 127 of /var/www/beta.dosomething.org/releases/20141209142557/lib/modules/dosomething/dosomething_helpers/dosomething_helpers.theme.inc).
````