travis: use clang's -fsanitize=address to catch more bugs

travis: use clang's -fsanitize=address to catch more bugs

The clang address sanitizer is able to catch quite a number of
memory-related bugs, such add memory leaks and buffer under/overruns.
So, enable the address sanitizer for one openssl and one mbedtls build.

This would have caught the buffer list unittest memory leak that
<1512724338-22197-1-git-send-email-steffan@karger.me> wants to fix.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1514540857-19290-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16102.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7b11915ddfe97d8c28f998db54c40384a4eafb93)