Fix crash when history pager is closed before search

With

	bind ctrl-r 'sleep 1' history-pager

typing ctrl-r,escape crashes fish in the history pager completion callback,
because the history pager has already been closed.

Prior to 55fd43d86c (Port reader, 2023-12-22), the completion callback
would not crash open a pager -- which causes weird races with the
user input.

Apparently this crash as been triggered by running "playwright",
and -- while that's running typing ctrl-r ligh escape.
Those key strokes were received while the kitty keyboard protocol
was active, possibly a race.

Fixes #11355

src/reader.rs

@@ -5271,7 +5271,9 @@ fn fill_history_pager(
                 return; // Stale request.
             }
             let history_size = zelf.history.size();
-            let history_pager = zelf.history_pager.as_mut().unwrap();
+            let Some(history_pager) = zelf.history_pager.as_mut() else {
+                return; // Pager has been closed.
+            };
             assert!(result.range.start < result.range.end);
             *history_pager = result.range;
             zelf.pager.extra_progress_text =

tests/pexpects/bind.py

@@ -353,7 +353,7 @@ send("\x1b")
 expect_str("foo")
 send("\x1b[A")
 expect_str("bind escape 'echo foo'")
-sendline("")
+sendline("bind --erase escape")
 expect_prompt()
 
 send("    a b c d\x01")  # ctrl-a, move back to the beginning of the line
@@ -405,6 +405,11 @@ expect_prompt()
 sendline("commandline -f and")
 expect_prompt()
 
+sendline("bind ctrl-g 'sleep 1' history-pager")
+expect_prompt()
+send("\x07")  # ctrl-g
+send("\x1b[27u")  # escape, to close pager
+
 # Check that the builtin version of `exit` works
 # (for obvious reasons this MUST BE LAST)
 sendline("function myexit; echo exit; exit; end; bind ctrl-z myexit")