ui: bound the input history size.

Only the lower end was checked, so a large value threw bad_alloc and exited.

src/ui/root.h

@@ -54,6 +54,11 @@ public:
   typedef std::vector<std::string> InputHistoryCategory;
   typedef std::map<int, InputHistoryCategory> InputHistory;
 
+  // Every category holds this many entries, and the whole history is written to
+  // and read back from the session, so the useful range ends long before the
+  // point where resizing it becomes a memory problem.
+  static constexpr int max_input_history_size = 4096;
+
   Root();
 
   void                init(Control* c);