Avoid JavaScript error during page load in non-frame mode

In non-frame mode, parent.window.synopsis refers to the synopsis div
rather than the nonexistent frame.  Unfortunately, the script wrongly
assumes that if it exists it must be a frame, leading to an error where
it tries to access the nonexistent attribute 'replace' of an undefined
value (synopsis.location).

Closes #406

haddock-api/resources/html/haddock-util.js

@@ -229,7 +229,7 @@ function perform_search(full)
 }
 
 function setSynopsis(filename) {
-    if (parent.window.synopsis) {
+    if (parent.window.synopsis && parent.window.synopsis.location) {
         if (parent.window.synopsis.location.replace) {
             // In Firefox this avoids adding the change to the history.
             parent.window.synopsis.location.replace(filename);