Reject unwanted inputs in "Writing harnesses" example

We could improve the C/C++ example provided here.

We could turn the following code:

    // Ensure exactly 2 4-byte numbers (numerator and denominator) are read
    if(size != 2 * sizeof(uint32_t)){
        return 0;
    }

To something like:

    // Ensure exactly 2 4-byte numbers (numerator and denominator) are read
    if(size != 2 * sizeof(uint32_t)){
        return -1;  // Reject; The input will not be added to the corpus.
    }

As documented here. This makes the fuzzing harness more efficient, and documents the "reject input" feature of libFuzzer. We should then explain the return -1; and drop a link to the libFuzzer docs in the following paragraph.

Start at the “Beyond byte arrays” section of the Writing harnesses example and compare its size check with the linked libFuzzer documentation on rejecting unwanted inputs. Update the example and following paragraph so the rejection return value is explained and the libFuzzer link is included.