ext2: Somewhat finish proofs
diff --git a/src/filo-fs-ext2.ads b/src/filo-fs-ext2.ads
index c48ee93..1757da8 100644
--- a/src/filo-fs-ext2.ads
+++ b/src/filo-fs-ext2.ads
@@ -24,7 +24,7 @@
       Success     : out    Boolean)
    with
       Pre => Is_Mounted (State),
-      Post => Success = Is_Open (State);
+      Post => (if Success then Is_Open (State) else Is_Mounted (State));
 
    procedure Close (State : in out T)
    with
@@ -37,7 +37,7 @@
       Buf      :    out Buffer_Type;
       Len      :    out Natural)
    with
-      Pre => Is_Open (State),
+      Pre => Is_Open (State) and Buf'Length > 0,
       Post => Is_Open (State);
 
 private