email: prevent NPE in EmailProvider

Check that we have loaded a valid cursor. After deleting an account some query could
return a null cursor.

Change-Id: Ief2f948a61a308d9e17af80730612a02306336e5
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge "email: prevent NPE in EmailProvider" into du44