svn merge -g crashes if there is no client side mergeinfo.
svn merge -g crashes if there is no client side mergeinfo.

* subversion/libsvn_client/log.c
  (svn_client__suggest_merge_sources): 
   Fix segfault if 'svn_client_get_mergeinfo' sets NULL mergeinfo 
   hash (implying no svn:mergeinfo prop).

Approved by: dlr

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion