build: use BOOST_MULTI_INDEX_ENABLE_SAFE_MODE when debugging

Use of this macro enables precondition checks for iterators and
functions of the library. It's use is recommended in debug builds.

See:
https://www.boost.org/doc/libs/1_78_0/libs/multi_index/doc/tutorial/debug.html.

configure.ac

@@ -1455,6 +1455,10 @@ if test "$use_boost" = "yes"; then
   dnl we don't use multi_index serialization
   BOOST_CPPFLAGS="$BOOST_CPPFLAGS -DBOOST_MULTI_INDEX_DISABLE_SERIALIZATION"
 
+  if test "$enable_debug" = "yes" || test "$enable_fuzz" = "yes"; then
+    BOOST_CPPFLAGS="$BOOST_CPPFLAGS -DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE"
+  fi
+
   if test "$suppress_external_warnings" != "no"; then
     BOOST_CPPFLAGS=SUPPRESS_WARNINGS($BOOST_CPPFLAGS)
   fi