On single group screens, uses default avatar as a fallback in group navigation menu

In case the WordPress Discussion Avatar display setting is disabled, using bp_group_current_avatar() to set group navigation menu was outputing an unfound image making the group edit screens unavailable. This fix uses default avatar as a fallback if no group avatar has been found to build the group navigation menu. It also keeps  bp_group_current_avatar() function in case it is still used by a plugin or a theme, but this function now outputs the current avatar.

Fixes #5343

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7745 cdf35c40-ae34-48e0-9cc9-0c9da1808c22