modern: Add error checking for enough tokens.

modern.slices had a bug in find-from* where it was taking the length of
a quotation instead of the original string.

extra/modern/modern.factor

@@ -138,12 +138,16 @@ MACRO:: read-matched ( ch -- quot: ( n string tag -- n' string slice' ) )
         [ ")" sequence= ]
     } 1|| ;
 
+ERROR: expected-length-tokens n string length seq ;
+: ensure-no-false ( n string seq -- n string seq )
+    dup [ length 0 > ] all? [ [ length ] keep expected-length-tokens ] unless ;
+
 ERROR: token-expected n string obj ;
 ERROR: unexpected-terminator n string slice ;
 : read-lowercase-colon ( n string slice -- n' string lowercase-colon )
     dup [ char: \: = ] count-tail
     '[
-        _ [ lex-factor ] replicate dup [ token-expected ] unless
+        _ [ lex-factor ] replicate ensure-no-false dup [ token-expected ] unless
         dup terminator? [ unexpected-terminator ] when
     ] dip swap 2array ;
 
@@ -214,6 +218,7 @@ ERROR: no-backslash-payload n string slice ;
         dup [ char: \\ = ] count-tail
         '[
             _ [ skip-blank-from slice-til-whitespace drop ] replicate
+            ensure-no-false
             dup [ no-backslash-payload ] unless
         ] dip swap 2array
     ] when ;

extra/modern/slices/slices.factor

@@ -63,7 +63,7 @@ ERROR: unexpected-end n string ;
     next-char-from 2nip ;
 
 : find-from* ( ... n seq quot: ( ... elt -- ... ? ) -- ... i elt ? )
-    [ find-from ] keep
+    [ find-from ] 2keep drop
     pick [ drop t ] [ length -rot nip f ] if ; inline
 
 : skip-blank-from ( n string -- n' string )