MDEV-7883 Segmentation failure when running mysqladmin -u root -p

take into account that argc can be 0
(if there were no commands on the command line)

client/mysqladmin.cc

@@ -1198,6 +1198,9 @@ static int execute_commands(MYSQL *mysql,int argc, char **argv)
 static char **mask_password(int argc, char ***argv)
 {
   char **temp_argv;
+  if (!argc)
+    return NULL;
+
   temp_argv= (char **)(my_malloc(sizeof(char *) * argc, MYF(MY_WME)));
   argc--;
   while (argc > 0)

mysql-test/t/mysqladmin.test

@@ -45,3 +45,9 @@ EOF
 --exec $MYSQLADMIN --defaults-extra-file=$MYSQLTEST_VARDIR/tmp/cnf_file -uroot -S $MASTER_MYSOCK -P $MASTER_MYPORT ping 2>&1
 
 --remove_file $MYSQLTEST_VARDIR/tmp/cnf_file
+
+#
+# MDEV-7883 Segmentation failure when running mysqladmin -u root -p
+#
+--error 1
+--exec $MYSQLADMIN -u root -p 2>&1 > /dev/null