[sanitizer] move a pointer dereference after its null check

[yingcong-wu]

Summary

The dereference of pointer ctx is not protected by its null check, this could be problematic.

[cchen15]

LGTM.

[MaskRay]

Nit

compiler-rt/lib/sanitizer_common/sanitizer_common_syscalls.inc

1376–1377

if (res >= 0 && ctx) to avoid nesting.

[yingcong-wu]

• avoid nesting

compiler-rt/lib/sanitizer_common/sanitizer_common_syscalls.inc

 
 POST_SYSCALL(flock)(long res, long fd, long cmd) {}
 
 PRE_SYSCALL(io_setup)(long nr_reqs, void **ctx) {
   if (ctx)
     PRE_WRITE(ctx, sizeof(*ctx));
 }
 
 POST_SYSCALL(io_setup)(long res, long nr_reqs, void **ctx) {
-  if (res >= 0) {
-    if (ctx)
-      POST_WRITE(ctx, sizeof(*ctx));
+  if (res >= 0 && ctx) {
     // (*ctx) is actually a pointer to a kernel mapped page, and there are
     // people out there who are crazy enough to peek into that page's 32-byte
     // header.
     if (*ctx)
       POST_WRITE(*ctx, 32);
   }
 }