lib/fold: fix warning with -DDEBUG

When compiling the fold code with DEBUG defined, we get:

../lib/fold/fold.c: In function 'fold_text':
../lib/fold/fold.c:37:3: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
   assert(bytes >= 0);
   ^

We should be comparing with != (size_t)-1, not >= 0

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>