Fix a possible warning without debugging.

Fix a possible warning without debugging.

src/alloc-slab.c (slab_unalloc): If assert() is #define'd to be empty,
    then size is unused.  Include a (void)size to quash a warning.
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-10