Add NULL checks

Add NULL checks

Extra NULL checks were added after malloc() calls to display out-of-
memory error and try to exit gracefully.

Function msica_op_create_*() now return NULL in out-of-memory condition
too. Since their output is directly used in msica_op_seq_add_head() and
msica_op_seq_add_tail() functions, later were extended to check for NULL
pointer arguments.

This patch follows Gert's recommendations from [openvpn-devel].

Signed-off-by: Simon Rozman <simon@rozman.si>
Message-ID: <20190117155829.GA92142@greenie.muc.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190224181400.42524-1-simon@rozman.si
20190117155829.GA92142@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18231.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>