[loose parser] Fetch token before comment when tokenizer raises unterminated comment error

Closes #197

acorn.js

@@ -242,6 +242,7 @@
       tokExprAllowed = !!exprAllowed;
       skipSpace();
     };
+    getToken.current = function() { return new Token(); };
     getToken.options = options;
     return getToken;
   };

acorn_loose.js

@@ -105,6 +105,8 @@
             replace = {start: e.pos, end: pos,
                        type: input.charAt(e.pos) == "`" ? tt.template : tt.templateContinued,
                        value: input.slice(e.pos + 1, pos)};
+          } else if (/comment/.test(msg)) {
+            replace = fetchToken.current();
           } else {
             replace = false;
           }