The regression test in test/asan/TestCases/heap-overflow-large.cc returns an exit code with this statement:
return res ? res : 1;
with the intention of always returning a non-zero exit code, no matter what res is (it's a result of a pretty much random memory read). However, when res is accidentally a multiple of 256, then (at least on Darwin) this will be treated as a zero exit code. Let's change the test to only return one byte of res and not the whole int.