[compiler-rt] [SanitizerCommon] add null check for fopen64 interceptor (PR #68760)

[Wu Yingcong via llvm-commits]

yingcong-wu wrote:

My test is not well-written. I don't know how fopen64() is used in other systems, so I don't know if I can test fopen64() directly.

https://github.com/llvm/llvm-project/pull/68760