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

[Thurston Dang via llvm-commits]

================
@@ -0,0 +1,8 @@
+// Check that fopen64(NULL, "r") is ok.
+
----------------
thurstond wrote:

One of your earlier changes removed the 'RUN' line, so this test will have an error such as
```
******************** TEST 'SanitizerCommon-lsan-x86_64-Linux :: fopen64_nullptr.c' FAILED ********************
Test has no 'RUN:' line
```

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