Fix for systems that have not all disks installed

First of all thanks for this great plugin.
We use a QNAP TS-873U-RP with currently 5 disks installed. The plugin throws errors with the hdtemp check because it receives as value for the empty hdd slots “–” which causes a syntax error.
To fix this, I patched the code so that the “empty” values are replaced with 0 and counted. After the SNMP requests the counted “empty” values are subtracted of the total amount of hdd slots to keep the result genuine. The complete “HD Temperature” code block can be found under http://localhost/directory/Plugins/Hardware/Storage-Systems/Others/check_qnap/review