(based on suggested patch by Jan-Benedict Glaw):
> While hacking mpd, I noticed that an assert()ion in xrealloc is wrong.
> A null size is perfectly legal, so we shouldn't assert on that.

Since some C libraries return NULL when size == 0, we'll make
sure we get a free()-able pointer since some of those C
libraries also barf on free(NULL).

git-svn-id: https://svn.musicpd.org/mpd/trunk@4740 09075e82-0dd4-0310-85a5-a0d7c8717e4f