Makefile: Replace "-fsanitize=address" with "-fsanitize=leak"

The Address Sanitizer is a debugging feature, not a security feature -
it has security issues:  <http://seclists.org/oss-sec/2016/q1/363>
Therefore, it should only be used when debugging.  Replace it with
"-fsanitize=leak", which is needed since -fsanitize=address no longer
includes it implicitly.

* Makefile (DEBUG): Add "-fsanitize=address".
  (ALL_SANITIZE_OPTIONS): Replace "-fsanitize=address" with
                          "-fsanitize=leak".