Handle NULL returns from calloc() in sample plugins.

Handle NULL returns from calloc() in sample plugins.

This is basic housekeeping, adding NULL checks to context initialization
of the sample plugin collection which are missing it.  Realistically,
this can never happen, but since these are supposed to be "good examples",
not checking calloc() return isn't one.

Trac: #587

Reported-By: Dogbert (in Trac)
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20200909104837.6123-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20922.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a61c08a2c80d95dcc2bc30ddcb9a54a462e565ed)