BUG/MINOR: mworker/cli: relative pid prefix not validated anymore

Since e8422bf ("MEDIUM: global: remove the relative_pid from global and
mworker"), the relative pid prefix is not tested anymore on the master
CLI. Which means any value will fall into the "1" process.

Since we removed the nbproc, only the "1" and the "0" (master) value are
correct, any other value should return an error.

Fix issue #1793.

This must be backported as far as 2.5.

src/cli.c

@@ -2294,6 +2294,9 @@ static int pcli_prefix_to_pid(const char *prefix)
 		if (proc_pid == 0) /* return the master */
 			return 0;
 
+		if (proc_pid != 1) /* only the "@1" relative PID is supported */
+			return -1;
+
 		/* chose the right process, the current one is the one with the
 		 least number of reloads */
 		list_for_each_entry(child, &proc_list, list) {