Avoid crashes under 'which-key-mode'

* src/keyboard.c (Fthis_single_command_keys): Don't allow calls to
Fvector with negative first argument.  (Bug#64857)

(cherry picked from commit 65834b8f8d)

src/keyboard.c

@@ -11116,8 +11116,8 @@ the command loop or by `read-key-sequence'.
 The value is always a vector.  */)
   (void)
 {
-  return Fvector (this_command_key_count
-		  - this_single_command_key_start,
+  ptrdiff_t nkeys = this_command_key_count - this_single_command_key_start;
+  return Fvector (nkeys < 0 ? 0 : nkeys,
 		  (XVECTOR (this_command_keys)->contents
 		   + this_single_command_key_start));
 }