Avoid an undefined behaviour identified by GCC's sanitizer that occurs
after svn_cache__set() with value==NULL to invalidate the dag cache.

* subversion/libsvn_subr/cache-inprocess.c
  (inprocess_cache_get_internal): Avoid passing NULL to memcpy.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632540 13f79535-47bb-0310-9956-ffa450edef68