Indeed, asan will generally miss bugs in third party libraries which are not compiled. But for memset and a few other popular libc functions asan has intercept

Indeed, asan will generally miss bugs in third party libraries which are not compiled. But for memset and a few other popular libc functions asan has interceptors, so in your example the bug could be reported. Currently, when dealing with memset, asan analyzes only the first and the last accessed byte, so in your example a[99] should be inside a redzone, otherwise the bug may still be missed. For heap, asan uses 128 byte redzones by default, but for stack -- redzones are only 32-byte.