test_src_input_stream random data triggering random failure

As experienced here https://code.videolan.org/fcartegnie/vlc/-/jobs/918078 Test randomly failing as the stream went, in that case, into infinite decompression error loop

FAIL: test_src_input_stream
===========================
[000055e0883a49a0] inflate stream error: unhandled decompression error (2)
[000055e0883a49a0] inflate stream error: unhandled decompression error (2)
[000055e0883a49a0] inflate stream error: unhandled decompression error (2)
[000055e0883a49a0] inflate stream error: unhandled decompression error (2)
[000055e0883a49a0] inflate stream error: unhandled decompression error (2)

Magic bytes being generated in:

#ifndef TEST_NET
static void
fill_rand( int i_fd, size_t i_size )
{
    uint8_t p_buf[4096];
    size_t i_written = 0;
    while( i_written < i_size )
    {
        size_t i_tocopy = __MIN( i_size - i_written, 4096 );

        vlc_rand_bytes(p_buf, i_tocopy);
        ssize_t i_ret = write( i_fd, p_buf, i_tocopy );
        assert( i_ret > 0 );
        i_written += i_ret;
    }
    assert( i_written == i_size );
}
#endif