Update check_mem.pl memory array values to match output of free from procps-ng 3.3.17 and 4.04

[VGerris]

Tested on Ubuntu 22.04.
I noticed when running the check, the swap values were incorrect on a machine with no swap. After investigating it turned out the values of the output seem to be mapped incorrectly ( for mentioned versions ).

This PR corrects that.

[VGerris]

Tested :

root@gitlab-server:/usr/lib/nagios/plugins# free -mt
               total        used        free      shared  buff/cache   available
Mem:            7937        4403         173          16        3360        3213
Swap:           2047           1        2046
Total:          9985        4404        2220

root@gitlab-server:/usr/lib/nagios/plugins# ./check_mem.pl -w 90,25 -c 95,50
OK: Memory Usage (W> 90, C> 95): 56% <br>Swap Usage (W> 25, C> 50): 0%|MemUsed=56%;90;95 SwapUsed=0%;25;50

root@gitlab-server:/usr/lib/nagios/plugins# swapoff /mnt/swapfile
root@gitlab-server:/usr/lib/nagios/plugins# free -mt
               total        used        free      shared  buff/cache   available
Mem:            7937        4420         174          17        3342        3195
Swap:              0           0           0
Total:          7937        4420         174
root@gitlab-server:/usr/lib/nagios/plugins# ./check_mem.pl -w 90,25 -c 95,50
OK: Memory Usage (W> 90, C> 95): 56% <br>Swap Usage (W> 25, C> 50): 0%|MemUsed=56%;90;95 SwapUsed=0%;25;50

Printed mapping of the memory array in the script :

0 : total
1 : used
2 : free
3 : shared
4 : buff/cache
5 : available
6 : Mem:
7 : 7937
8 : 4411
9 : 168
10 : 16
11 : 3358
12 : 3205
13 : Swap:
14 : 2047
15 : 1
16 : 2046
17 : Total:
18 : 9985
19 : 4412
20 : 2215

So before the change, swap would be calculated as the total of mem and swap :

root@gitlab-server:/usr/lib/nagios/plugins# ./check_mem.pl -w 90,25 -c 95,50
<b>CRITICAL: Memory Usage (W> 90, C> 95): 56% <br>Swap Usage (W> 25, C> 50): 56%</b>|MemUsed=56%;90;95 SwapUsed=56%;25;50