monitor: Fix file_completion() to check for stat() failure

stat() can fail for a file name just read with readdir().  Easiest way
to trigger is a dangling symbolic link --- look ma, no race!  When it
fails, file_completion() uses sb.st_mode uninitialized.  If the
directory bit happens to be set, it appends a "/" to the completed
name.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

monitor.c

@@ -4207,9 +4207,9 @@ static void file_completion(const char *input)
             /* stat the file to find out if it's a directory.
              * In that case add a slash to speed up typing long paths
              */
-            stat(file, &sb);
-            if(S_ISDIR(sb.st_mode))
+            if (stat(file, &sb) == 0 && S_ISDIR(sb.st_mode)) {
                 pstrcat(file, sizeof(file), "/");
+            }
             readline_add_completion(cur_mon->rs, file);
         }
     }