ra_serf: Add non NULLness check before checking the returned error. Hey, we might well have no error at all.
ra_serf: Add non NULLness check before checking the returned error. Hey, we
might well have no error at all.

* subversion/libsvn_ra_serf/locks.c
  (svn_ra_serf__lock): Check if there really was an error.

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