when shrinking, always wipe memory to zero even if we don't actually

truncate the file

src/mtagmap.lisp

@@ -191,7 +191,7 @@
       (declare (type mindex next))
       (when (> (the mindex (+ next bytes)) (the mindex len))
 	(mtagmap-extend-alloc mtagmap bytes))
-      (incf (mtagmap-next mtagmap) bytes)
+      (setf (mtagmap-next mtagmap) (the mindex (+ next bytes)))
       next)))
 
 (defun mtagmap-check-read (mtagmap)
@@ -236,12 +236,12 @@
   (mtagmap-check mtagmap)
   (let* ((next (mtagmap-next mtagmap))
 	 (bytes (round-up-to-pagesize next)) (file-len (mtagmap-file-length mtagmap)))
+    (assert (>= file-len bytes))
+    (unless (= next bytes)
+      (osicat-posix:memset (cffi:inc-pointer (mtagmap-ptr mtagmap) next)
+			   0 (- bytes next)))
     (unless (= bytes file-len)
-      (assert (> file-len bytes))
       (assert (>= bytes next))
-      (unless (= next bytes)
-	(osicat-posix:memset (cffi:inc-pointer (mtagmap-ptr mtagmap) next)
-			     0 (- bytes next)))
       (mtagmap-resize mtagmap bytes))))
 
 (defun mtagmap-schema (mtagmap)