Fix underflow in commandline jump functions

This patch fixes an underflow in the jump family of readline commands
when called via `commandline -f` outside of a bind context such as
`commandline -f backward-jump`. To reproduce, run that command at a
prompt and the shell will crash with a buffer underlow.

This happens because the jump commands have non-zero arity, requiring a
character event to be pushed on the function args stack. Pushing the
character event is handled in `function_push_args`, called by
`inputter_t::mapping_execute`, which checks the arity of the function
and enqueues the required number of charcter events. However,
`builtin_commandline` calls `reader_queue_ch`, which in turn calls
`inputter_t::queue_ch`, which immediately enqueues the readline event
without calling `function_push_args`, so the character event is never
pushed on the arg stack.

This patch adds a check in inputter_t::queue_ch which checks if the
character event is a readline event, and if so, calls
`function_push_args`.

src/input.cpp

@@ -433,7 +433,12 @@ bool inputter_t::mapping_is_match(const input_mapping_t &m) {
     return true;
 }
 
-void inputter_t::queue_ch(const char_event_t &ch) { event_queue_.push_back(ch); }
+void inputter_t::queue_ch(const char_event_t &ch) {
+    if (ch.is_readline()) {
+        function_push_args(ch.get_readline());
+    }
+    event_queue_.push_back(ch);
+}
 
 void inputter_t::push_front(const char_event_t &ch) { event_queue_.push_front(ch); }