bpf: initialize the verifier log

bpf: initialize the verifier log

If program loading fails before verifier prints its first
message, the verifier log will not be initialized.  Always
set the first character of the log buffer to zero to make
sure we don't dump non-printable characters to the terminal.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>