Fix unchecked return value to prevent VSCode compiler errors

This error was reported by a student, who noticed when compiling in VSCode, the following error message was generated:

```
threadpool_lib.c: In function 'count_number_of_threads':
threadpool_lib.c:46:9 error: ignoring return value of 'fgets', declared with attribute warn_unused_result [-Werror=unused-result]
    fgets(buf, sizeof buf, p);
    ^
cc1: all warnings treated as errors
<builtin>: recipe for target 'threadpool_lib.o' failed
make: *** [threadpool_lib.o] Error 1
```

I was not able to reproduce this problem in `clang` or `gcc` on my machine, but I figured it was worth the PR. We should be checking return values anyways.

See merge request !4