Fix process_result locking by acquiring in process_wait as well to prevent freeing memory too early

src/threads/thread.h

@@ -34,9 +34,10 @@ typedef int tid_t;
 /* A process result, synchronised between parent and child. */
 struct process_result
 {
-  tid_t tid;        /* The tid of the child process. */
-  int exit_status;  /* The exit status of the child process. Initially set to
-                       -1, then to exit_status when child dies. */
+  tid_t tid;             /* The tid of the child process. */
+  int exit_status;       /* The exit status of the child process. Initially set
+                            to -1, then to exit_status when child dies. */
+  struct lock lock;      /* Lock the exit_status and sema. */
   struct semaphore sema; /* Semaphore to signal the parent that the exit_status
                             has been set. */
   struct list_elem elem; /* List element for the parent's children list. */