Replaced atoi() with strtol() for better error handling.

If atoi() fails to convert, '0' is returned which matches the root user affecting the search group pool.

Consider adding range checks for strtol() return.

--{ Output sample:
```
shell@android:/data/local/tmp $ ./charm -g foo                                 
[*] uid=2000(shell), groups=1003(graphics),1004(input),1007(log),1009(mount),1011(adb),1015(sdcard_rw),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats),2000(shell),0(root)
[*] Found 0 entries that are set-uid executable
[*] Found 0 entries that are set-gid executable
[*] Found 0 entries that are writable
shell@android:/data/local/tmp $
shell@android:/data/local/tmp $
shell@android:/data/local/tmp $ ./charm_patch -g foo                           
[!] Unknown group: foo
[*] uid=2000(shell), groups=1003(graphics),1004(input),1007(log),1009(mount),1011(adb),1015(sdcard_rw),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats),2000(shell)
[*] Found 0 entries that are set-uid executable
[*] Found 0 entries that are set-gid executable
[*] Found 0 entries that are writable
```