prompt: prevent execution of inadmissible functions in view mode

This fixes https://savannah.gnu.org/bugs/?62912.

Bug existed since commit 958ec294 from earlier today,
but was enabled by commit 433dd921 from three days ago.

src/prompt.c

@@ -367,9 +367,12 @@ int do_statusbar_input(bool *finished)
 		}
 #endif
 		else {
-			/* Handle some other shortcut, and indicate that we're done. */
+			/* Handle any other permissible shortcut, and stamp as done. */
-			function();
+			if (!ISSET(VIEW_MODE) || !changes_something(function)) {
-			*finished = TRUE;
+				function();
+				*finished = TRUE;
+			} else
+				beep();
 		}
 	}
 

src/prototypes.h

@@ -427,6 +427,7 @@ void terminal_init(void);
 void confirm_margin(void);
 #endif
 void unbound_key(int code);
+bool changes_something(const void *f);
 void inject(char *burst, size_t count);
 
 /* Most functions in prompt.c. */