Free Entity before replacing it from the cache

Toward the beginning of XrdSecProtocolgsi::Authenticate(), memory may be
allocated for the Entity with strdup():
https://github.com/xrootd/xrootd/blob/a76eccc1f2/src/XrdSecgsi/XrdSecProtocolgsi.cc#L1873

Later in the same function, the Entity may be replaced with an entry from the
cache:
https://github.com/xrootd/xrootd/blob/a76eccc1f2/src/XrdSecgsi/XrdSecProtocolgsi.cc#L1951

However, the memory allocated with strdup() is not free()d. Call FreeEntity()
before replacing Entity with a copy.

src/XrdSecgsi/XrdSecProtocolgsi.cc

@@ -1948,6 +1948,7 @@ int XrdSecProtocolgsi::Authenticate(XrdSecCredentials *cred,
          } else {
             // Fetch a copy of the saved entity
             int slen = 0;
+            FreeEntity(&Entity);
             CopyEntity((XrdSecEntity *) cent->buf1.buf, &Entity, &slen);
             // Notify
             DEBUG("Got Entity from cacheAuthzFun ("<<slen<<" bytes)");