hashtables: fix the error thrown by H{ 1 }.

core/hashtables/hashtables-tests.factor

@@ -1,5 +1,5 @@
 USING: accessors assocs continuations fry hashtables kernel make
-math namespaces sequences tools.test ;
+math namespaces sequences tools.test eval ;
 
 { H{ } } [ { } [ dup ] H{ } map>assoc ] unit-test
 
@@ -125,6 +125,9 @@ H{ } "x" set
 ! non-integer capacity not allowed
 [ 0.75 <hashtable> ] must-fail
 
+[ "H{ 1 }" eval( str -- obj ) ] must-fail
+[ "H{ 1 2 -- 3 }" eval( str -- obj ) ] must-fail
+
 ! Another crash discovered by erg
 { } [
     H{ } clone

core/hashtables/hashtables.factor

@@ -194,7 +194,13 @@ M: hashtable assoc-like
 ERROR: malformed-hashtable-pair seq pair ;
 
 : check-hashtable ( seq -- seq )
-    dup [ dup length 2 = [ drop ] [ malformed-hashtable-pair ] if ] each ;
+    dup [
+        dup sequence? [
+            dup length 2 = [ drop ] [ malformed-hashtable-pair ] if
+        ] [
+            malformed-hashtable-pair
+        ] if
+    ] each ;
 
 : parse-hashtable ( seq -- hashtable )
     check-hashtable H{ } assoc-clone-like ;