Use unsigned variables for storing the count of items or for iteration
variables.  Since there can never be a negative number of items, it
makes sense to use an unsigned data type here.  This change is safe
because the unsigned values are only used for adddressing array items.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7214 09075e82-0dd4-0310-85a5-a0d7c8717e4f