Address more integer-related compiler warnings.  No functional change.

* libsvn_subr/cache-membuffer.c
  (free_spare_group): silence signedness warning
* libsvn_subr/prefix_string.c
  (svn_prefix_string__create): cast is safe as there are < 255 sub-nodes
* libsvn_subr/sorts.c
  (heap_is_less): cast to greater type, safely ignoring signedness



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543635 13f79535-47bb-0310-9956-ffa450edef68