input: Let function_pop_arg return an Option

This would crash if you ran `commandline -f backward-jump`.

The C++ version would read a char (but badly), this doesn't anymore.

So, at least instead of crashing, just do nothing.

src/input.rs

@@ -495,10 +495,9 @@ impl Inputter {
         self.input_function_args.push(arg);
     }
 
-    pub fn function_pop_arg(&mut self) -> char {
+    pub fn function_pop_arg(&mut self) -> Option<char> {
         self.input_function_args
             .pop()
-            .expect("function_pop_arg underflow")
     }
 
     fn function_push_args(&mut self, code: ReadlineCmd) {

src/reader.rs

@@ -3072,10 +3072,11 @@ impl ReaderData {
                     _ => unreachable!(),
                 };
                 let (elt, _el) = self.active_edit_line();
-                let target = self.inputter.function_pop_arg();
-                let success = self.jump(direction, precision, elt, target);
+                if let Some(target) = self.inputter.function_pop_arg() {
+                    let success = self.jump(direction, precision, elt, target);
 
-                self.inputter.function_set_status(success);
+                    self.inputter.function_set_status(success);
+                }
             }
             rl::RepeatJump => {
                 let (elt, _el) = self.active_edit_line();

tests/pexpects/bind.py

@@ -372,6 +372,10 @@ send('\x07') # ctrl-g
 send('\r')
 expect_prompt("foobar")
 
+# This should do nothing instead of crash
+sendline("commandline -f backward-jump")
+expect_prompt()
+
 # 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")