input: beep when invalid key is pressed at yesno prompt or in linter menu

Just like nano beeps when an unbound key is pressed at other prompts,
or in the help viewer, or in the file browser.

Signed-off-by: Brand Huntsman <alpha@qzx.com>

src/prompt.c

@@ -763,6 +763,8 @@ int do_yesno_prompt(bool all, const char *msg)
 			}
 		}
 #endif /* ENABLE_MOUSE */
+		else
+			beep();
 	}
 
 	return choice;

src/text.c

@@ -3008,7 +3008,8 @@ void do_linter(void)
 				last_wait = time(NULL);
 				statusline(NOTICE, curlint->msg);
 			}
-		}
+		} else
+			beep();
 	}
 
 	for (curlint = lints; curlint != NULL;) {