Check return value of ms_error_text()

Check return value of ms_error_text()

ms_error_text() may return NULL, and it is unclear (or, at least
undocumented) whether the OpenSSL ERR code (and our code using the ERR
code) can deal with esd->string being NULL.  So, just to be sure, check
that ms_error_text() succeeded before passing the result to
ERR_load_strings().

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <561130FC.8090008@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10176
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 5584b738a332d0abc740d9303c275764c2ca13f1)