fix a segfault caused by passing a non-null buffer to bpf_prog_load

[snowp]

During error handling bcc will attempt to write the message to the
provided buffer. It checks for a provided buffer either by checking the
log_buf_size parameter or by checking log_buf to see if it's non-null.

Because of the inconsistent checks on bcc's side and the specific
parameters that rust-bcc passes, this is only a problem when the call to
bpf_prog_load returns certain errors.

The flow that causes the segfault looks like this: rust-bcc sets log_level=0,log_size=0
as the initial parameters. bcc will issue the syscall, but upon failure will reissue
the bcc_prog_load_xattr with log_level set to 1. At this point, the same error will be
produced and outputted to a new, temporary buffer (since the provided
log_size is 0). Following this, it will attempt to log to the user provided
buffer as log_level is now > 0, checking to see if the provided buffer
is non-null. As rust-bcc provided a non-null ptr, bcc attempts to write
to a zero buffer which causes a SIGSEGV.

Relevant links:
Checks for log_buf_size:
https://github.com/iovisor/bcc/blob/d147588ebe35b7cd2b4d253a7da18bef253ea78d/src/cc/libbpf.c#L519

Checks for log_buf:
https://github.com/iovisor/bcc/blob/d147588ebe35b7cd2b4d253a7da18bef253ea78d/src/cc/libbpf.c#L641

[brayniac]

Awesome! Thanks for the PR!