Attempted fix for $c->user->set_avatar crashes.

Since I am not able to reproduce this, I'm inserting an extra check for
file existence before attempting the copy.

Really, store_avatar should only receive source filenames which exist
from files_in_stash but that appears not to be happening.
Merge pull request #148 from jbarrett/fix-userpage-avatar-crash

Attempted fix for $c->user->set_avatar crashes.