Add fail-safe in case of nvidia-settings segfault

[suburbski]

Now and then, nvidia-settings throws a segfault for whatever reason on my machine, which causes the script to set cur_t to something that is not a number, eventually freezing the whole OS.

kernel: [ 1652.744323] nvidia-settings[10112]: segfault at 7faf6e5f97 ip 00007faf6dffcee0 sp 00007ffcc781d520 error 4 in libgtk-3.so.0.2404.16[7faf6dee3000+36f000]
kernel: [ 1652.744329] Code: 84 a5 03 00 00 48 8b 31 48 8b 38 48 89 54 24 28 e8 95 f0 ee ff 48 8b 54 24 28 85 c0 79 ac 8b 4c 24 48 0f 1f 84 00 00 00 00 00 <f3> 0f 6f 0a 89 4c 24 08 0f 29 4c 24 50 48 8b 42 10 48 89 44 24 60
gpufancurve.desktop[2103]: Segmentation fault (core dumped)
gpufancurve.desktop[2103]: /home/suburbski/nfancurve/temp.sh: 116: [: Illegal number: 

This fix adds a check which prevents the OS freeze in my case and generally other problems caused by temperature reading faults by setting cur_t to ot in case the former is not a number.