dissector: ip_auth_hdr: Fix possible null pointer dereference

Fix an unconditional dereference of a pkt_pull() return value to prevent
dereferencing a null pointer.

This was found by the Coverity scanner.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dissector: icmpv6: Fix another possible null pointer dereference

This was missing in the previous commit 8b824423 ("dissector: icmpv6:
Fix possible null pointer dereferences"), so fix the possible null
pointer dereference now.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
flowtop: Remove unnecessary bug_on() and rework presenter_screen_init()

The screen pointer can never be NULL, since initscr() exits the process
in case of an error. Thus the bug_on() check can be omitted.

Also rework presenter_screen_init() - where initscr() is called - to
return the pointer.

This will also silence a Coverity scanner error (even though it isn't
actually one).

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>