warnings from hook_boot

dropbox_boot() accesses $arg[1] and $arg[2] without checking whether they exist first, which causes warnings like:

/user

Notice: Undefined offset: 1 in /var/www/audean.com/www/sites/blog.riff.org/modules/dropbox/dropbox.module on line 31
Warning: Cannot modify header information - headers already sent by (output started at (snip)/modules/dropbox/dropbox.module:31) in (snip)/includes/bootstrap.inc on line 630 (4 times)

/user/<uid>

Notice: Undefined offset: 2 in /var/www/audean.com/www/sites/blog.riff.org/modules/dropbox/dropbox.module on line 31
Warning: Cannot modify header information - headers already sent by (output started at (snip)/modules/dropbox/dropbox.module:31) in (snip)/includes/bootstrap.inc on line 630 (4 times)

Small patch suggested.