[searchcursor addon] Refuse to search for queries that match everything

It doesn't do anything useful, and makes replace-all loop
forever.

Closes #2259

addon/search/searchcursor.js

@@ -8,6 +8,10 @@
     pos = pos ? doc.clipPos(pos) : Pos(0, 0);
     this.pos = {from: pos, to: pos};
 
+    // Prevent nonsensical behavior from queries that match everything.
+    if (typeof query == "string" ? query == "" : query.test(""))
+      query = /x^/;
+
     // The matches method is filled in based on the type of query.
     // It takes a position and a direction, and returns an object
     // describing the next occurrence of the query, or null if no