Make the slave die if master is 5.0. Indeed, 5.0 masters send

a Format_description_log_event (or maybe it will be named
Description_log_event) which is not recognized by 4.0, so
a 4.0 can't be a slave of 5.0. We detect it early to produce
a helpful message instead of "corrupted relay log" later.

sql/slave.cc

@@ -1017,11 +1017,12 @@ static int check_master_version(MYSQL* mysql, MASTER_INFO* mi)
       BINLOG_FORMAT_323_GEQ_57 ;
     break;
   case '4':
-  case '5':
     mi->old_format = BINLOG_FORMAT_CURRENT;
     break;
   default:
-    errmsg = "Master reported unrecognized MySQL version";
+    /* 5.0 is not supported */
+    errmsg = "Master reported an unrecognized MySQL version. Note that 4.0 \
+slaves can't replicate a 5.0 or newer master.";
     break;
   }