avoid negative indices as argument ot drop

--- a/src/Pure/Tools/find_theorems.ML	Mon Dec 14 11:30:13 2009 +0000
+++ b/src/Pure/Tools/find_theorems.ML	Mon Dec 14 16:35:00 2009 +0100
@@ -409,7 +409,7 @@
 
         val len = length matches;
         val lim = the_default (! limit) opt_limit;
-      in (SOME len, drop (len - lim) matches) end;
+      in (SOME len, drop (Int.max (len - lim, 0)) matches) end;
 
     val find =
       if rem_dups orelse is_none opt_limit