proc: mem_release() should check mm != NULL

mem_release() can hit mm == NULL, add the necessary check.

Cc: stable@kernel.org
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -822,7 +822,7 @@ loff_t mem_lseek(struct file *file, loff_t offset, int orig)
 static int mem_release(struct inode *inode, struct file *file)
 {
 	struct mm_struct *mm = file->private_data;
-
+	if (mm)
 		mmput(mm);
 	return 0;
 }