Die if we sent --max-delete=0 to a receiving side that is too old.

compat.c

@@ -26,9 +26,11 @@ int remote_protocol = 0;
 
 extern int verbose;
 extern int am_server;
+extern int am_sender;
 extern int inplace;
 extern int fuzzy_basis;
 extern int read_batch;
+extern int max_delete;
 extern int checksum_seed;
 extern int basis_dir_cnt;
 extern int prune_empty_dirs;
@@ -75,6 +77,16 @@ void setup_protocol(int f_out,int f_in)
 		exit_cleanup(RERR_PROTOCOL);
 	}
 
+	if (protocol_version < 30) {
+		if (max_delete == 0 && am_sender) {
+			rprintf(FERROR,
+			    "--max-delete=0 requires protocol 30 or higher"
+			    " (negotiated %d).\n",
+			    protocol_version);
+			exit_cleanup(RERR_PROTOCOL);
+		}
+	}
+
 	if (protocol_version < 29) {
 		if (fuzzy_basis) {
 			rprintf(FERROR,