rpc: Add coverity[dead_error_begin] tag

Coverity misses the nuance of VIR_FREE(privkey) setting privkey = NULL when
if (!(virFileExists(privkey))) is true and thus declares the code dead.

src/rpc/virnetclient.c

@@ -430,6 +430,7 @@ virNetClientPtr virNetClientNewLibSSH2(const char *host,
                 VIR_FREE(privkey);
             /* DSA */
             if (!privkey) {
+                /* coverity[dead_error_begin] */
                 virBufferAsprintf(&buf, "%s/.ssh/id_dsa", homedir);
                 if (!(privkey = virBufferContentAndReset(&buf)))
                     goto no_memory;