Add comment explaining why buffer isn't overflowed

Author: pvdrz

src/shims/env.rs

@@ -137,7 +137,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 if (bytes.len() as u64) < size {
                     // We add a `/0` terminator
                     bytes.push(0);
-                    // This is ok because the buffer is larger than the path with the null terminator.
+                    // This is ok because the buffer was strictly larger than `bytes`, so after
+                    // adding the null terminator, the buffer size is larger or equal to
+                    // `bytes.len()`, meaning that `bytes` actually fit inside tbe buffer.
                     this.memory_mut()
                         .get_mut(buf.alloc_id)?
                         .write_bytes(tcx, buf, &bytes)?;