Add some error checking to CHAR:

core/parser/parser-tests.factor

@@ -498,3 +498,5 @@ DEFER: blah
 [ error>> error>> def>> \ blah eq? ] must-fail-with
 
 [ ] [ f lexer set f file set "Hello world" note. ] unit-test
+
+[ "CHAR: \\u9999999999999" eval ] must-fail

core/syntax/syntax.factor

@@ -62,7 +62,7 @@ IN: bootstrap.syntax
     "CHAR:" [
         scan {
             { [ dup length 1 = ] [ first ] }
-            { [ "\\" ?head ] [ next-escape drop ] }
+            { [ "\\" ?head ] [ next-escape >string "" assert= ] }
             [ name>char-hook get call ]
         } cond parsed
     ] define-syntax