Fix unbalanced parenthesis crash in Common Lisp lexer (#1129)

The Common Lisp lexer attempted to guard against an unmatched closing
parenthesis being used by checking whether the stack of states was
empty.

This misunderstood the way the stack is used. The stack should always
have at least one state (the bottom-most :root state). Popping this
state will cause an empty stack error to be thrown (as noted in #1102).

The correct guard is to check whether the size of the stack is 1. If it
is, then we have an unmatched closing parenthesis and should generate an
error token. This fixes #1102.

lib/rouge/lexers/common_lisp.rb

@@ -320,7 +320,7 @@ class complex concatenated-stream cons echo-stream file-stream
 
         rule /\(/, Punctuation, :root
         rule /\)/, Punctuation do
-          if stack.empty?
+          if stack.size == 1
             token Error
           else
             token Punctuation

spec/lexers/common_lisp_spec.rb

@@ -18,4 +18,12 @@
       assert_guess :mimetype => 'text/x-common-lisp'
     end
   end
+
+  describe 'lexing' do
+    include Support::Lexing
+
+    it 'does not crash on unbalanced parentheses' do
+      subject.lex(")\n").to_a
+    end
+  end
 end

spec/visual/samples/common_lisp

@@ -1,5 +1,8 @@
 #nil (commented (form ftw))
 
+; Unmatched closing parenthesis
+)
+
 ;;;; TYPEP und Verwandtes
 ;;;; Michael Stoll, 21. 10. 1988
 ;;;; Bruno Haible, 10.6.1989