Fix some read errors

src/kernel.sl

@@ -444,3 +444,6 @@
     (t (print obj)))
   (when newline
     (println)))
+
+(breakpoint)
+'

src/read.c

@@ -486,6 +486,9 @@ static LispVal *read_internal(struct ReadState *state) {
     case '\'': {
         popc(state); // '
         LispVal *tail = read_internal(state);
+        if (!tail) {
+            READ_ERROR(state, 1, "quote not quoting anything");
+        }
         LispVal *res = Fpair(Qquote, Fpair(tail, Qnil));
         refcount_unref(tail);
         refcount_unref(TAIL(res));