If donor loop receives unknown signal from the SST script it is an

error condition (SST failure), so it should set error code before
exiting.

Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>

sql/wsrep_sst.cc

@@ -1840,6 +1840,8 @@ wait_signal:
       else
       {
         WSREP_WARN("Received unknown signal: '%s'", out);
+        /* since it is the end of the loop, we must set error code */
+        err=-EINVAL;
         proc.wait();
       }
     }