SECURITY.md: added memory allocation error as a non security issue.
mruby expects `malloc(3)` returns `NULL` for too big allocations, so
even if big object allocation (e.g.  `[1,2,3]*268888888888888818`)
caused ASAN/Valgrind warnings, it's intentional, and we won't consider
the warning as a security issue.
@@ -15,4 +15,6 @@ We *don't* consider following issues as vulnerabilities:
* Runtime C undefined behavior (including integer overflow)
* Crash caused by misused API
-* Crash caused by tweaked compiled binary
+* Crash caused by modified compiled binary
+* ASAN/Valgrind warning for too big memory allocation
+ mruby assumes `malloc(3)` returns `NULL` for too big allocations
