Comment for #26196.

Fixes #26196.

Alternatively we could explicitly check and complain (eg. panic), but I don't see the value-add.

src/libstd/sys/windows/os.rs

@@ -311,6 +311,7 @@ impl ExactSizeIterator for Args {
 
 impl Drop for Args {
     fn drop(&mut self) {
+        // NULL-safe
         unsafe { c::LocalFree(self.cur as *mut c_void); }
     }
 }
@@ -321,6 +322,8 @@ pub fn args() -> Args {
         let lpCmdLine = c::GetCommandLineW();
         let szArgList = c::CommandLineToArgvW(lpCmdLine, &mut nArgs);
 
+        // cur may be NULL if CommandLineToArgvW failed,
+        // in which case the range is empty to prevent reads
         Args { cur: szArgList, range: 0..(nArgs as isize) }
     }
 }