06-onion.c: Fix value of 'sizeof' when allocating 'pthread_t' array with 'malloc'

[Bogdanisar]

Hello, I suspect that the project uses an improper sizeof value when calling malloc inside the tests file. We are creating an array of pthread_t so we should call malloc with sizeof(pthread_t), but we use the size of the pointer instead.

The execution might run fine because pthread_t and pthread_t * can have the same size (both are 8 bytes on my machine), but that is not necessarily true.

I think that we should change it.

[davidmoreno]

Thanks!