MOVE: do not crash when u is 0 (#126)

This commit adds ?DUP IF guards to MOVE and MOVE- that make them exit
when u is 0. Without these guards a DO loop was executed 65535 times
instead of 0.

In particular, this fixes a crash when "105 LOAD I " is executed
immediately after boot. Block editor word "I" passes IBUF length (0)
to MOVE- and MOVE in this case, causing a crash.

blk/370

@@ -1,15 +1,15 @@
 : MOVE ( a1 a2 u -- )
-    ( u ) 0 DO ( a1 a2 )
+    ?DUP IF ( u ) 0 DO ( a1 a2 )
         SWAP C@+ ( a2 a1+1 x )
         ROT C!+ ( a1+1 a2+1 )
-    LOOP 2DROP ;
+    LOOP THEN 2DROP ;
 : MOVE- ( a1 a2 u -- )
-    TUCK + 1- ( a1 u a2+u-1 )
+    ?DUP IF TUCK + 1- ( a1 u a2+u-1 )
     ROT 2 PICK + 1- ( u a2+u-1 a1+u-1 )
     ROT ( u ) 0 DO ( a2 a1 )
         C@- ( a2 a1-1 x )
         ROT C!- ( a1-1 a2-1 ) SWAP ( a2 a1 )
-    LOOP 2DROP ;
+    LOOP THEN 2DROP ;
 : MOVE, ( a u -- ) H@ OVER ALLOT SWAP MOVE ;
 : PREV 3 - DUP @ - ;