Fix a compiler warning.  No functional change.

* subversion/libsvn_subr/cache-membuffer.c
  (ensure_data_insertable_l1): Satisfy the -Wparentheses nanny.
    
    Perhaps we should just disable that particular warning flag...


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519275 13f79535-47bb-0310-9956-ffa450edef68