Fix a problem identified by valgrind when running mergeinfo-test 6.

* subversion/libsvn_subr/mergeinfo.c
  (svn_rangelist__combine_adjacent_ranges): Don't read beyond allocated memory.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1460763 13f79535-47bb-0310-9956-ffa450edef68