Rewrite sprintf_alloc.

Rewrite sprintf_alloc.

The example in printf(3) that this was taken from should not be used as
it ignores negative return codes from vsnprintf, instead allocating more
memory. Given an error from vsnprintf, this would loop until the process'
virtual memory is exhausted.

So we just exit in the event of a problem instead.

git-svn-id: http://opkg.googlecode.com/svn/trunk@581 e8e0d7a0-c8d9-11dd-a880-a1081c7ac358