Fix <c-r> use-after-free InsertCompletionHide touches used register

Before performing the insertion, InsertCompleter::insert calls
try_accept() to accept any selected completion candidate.  If there
is one, we fire InsertCompletionHide. If that one modifies the register
used by <c-r>, the inserted StringViews will be dangling.

Fix this by running try_insert first, and read from the register later.
Note that we call try_accept() twice but that's fine.

It would probably make more sense to copy the register before calling
insert() but I don't think it matters.

Closes #5220

src/input_handler.cc

@@ -15,6 +15,7 @@
 #include "window.hh"
 #include "word_db.hh"
 
+#include <concepts>
 #include <utility>
 #include <limits>
 
@@ -1310,7 +1311,7 @@ public:
                     auto cp = key.codepoint();
                     if (not cp or key == Key::Escape)
                         return;
-                    insert(RegisterManager::instance()[*cp].get(context()));
+                    insert([&] { return RegisterManager::instance()[*cp].get(context()); });
                 }, "enter register name", register_doc.str());
             update_completions = false;
         }
@@ -1430,6 +1431,13 @@ private:
         }, false);
     }
 
+    template<std::invocable Func>
+    void insert(Func&& lazy_strings)
+    {
+        m_completer.try_accept();
+        insert(std::forward<Func>(lazy_strings)());
+    }
+
     void insert(Codepoint key)
     {
         String str{key};

test/hooks/completion-hide-using-register/cmd

@@ -0,0 +1 @@
+

test/hooks/completion-hide-using-register/in

@@ -0,0 +1 @@
+echo

test/hooks/completion-hide-using-register/out

@@ -0,0 +1,2 @@
+echofoobar
+echo

test/hooks/completion-hide-using-register/rc

@@ -0,0 +1,8 @@
+set-option global autocomplete insert
+hook global InsertCompletionHide .+ %{
+    evaluate-commands -draft -save-regs '"' %{
+        select %val{hook_param}
+        set-register dquote foo bar
+        execute-keys <a-p>
+    }
+}

test/hooks/completion-hide-using-register/script

@@ -0,0 +1,7 @@
+ui_out -until '{ "jsonrpc": "2.0", "method": "refresh", "params": [true] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "Oe" ] }'
+ui_out -until '{ "jsonrpc": "2.0", "method": "menu_show", "params": [[[{ "face": { "fg": "default", "bg": "default", "underline": "default", "attributes": [] }, "contents": "echo" }, { "face": { "fg": "default", "bg": "default", "underline": "default", "attributes": [] }, "contents": " " }, { "face": { "fg": "cyan", "bg": "default", "underline": "default", "attributes": [] }, "contents": "out" }]], { "line": 0, "column": 0 }, { "fg": "white", "bg": "blue", "underline": "default", "attributes": [] }, { "fg": "blue", "bg": "white", "underline": "default", "attributes": [] }, "inline"] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "<c-n>" ] }'
+ui_out -until '{ "jsonrpc": "2.0", "method": "menu_select", "params": [1] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "<c-r>\"" ] }'
+ui_out -until '{ "jsonrpc": "2.0", "method": "menu_hide", "params": [] }'