allow -1 sound index in OPF_GAME_SND syntax check (#7540)

The numeric branch of the OPF_GAME_SND check in check_sexp_syntax never
actually ran until the type2 variable-overload bug was fixed, so missions
using a sound index of -1 ("no sound" / "default") loaded fine.  Now that
the check is live, explicitly allow -1, which the runtime already treats as
an invalid (unset) sound.  Also return the more descriptive
SEXP_CHECK_INVALID_GAME_SND / SEXP_CHECK_INVALID_FIREBALL error codes, and
let nested-operator (SEXP_ATOM_LIST) arguments fall through since their type
is already validated upstream.

Also route the Lua SEXP soundentry argument through sexp_get_sound_index so
it accepts a table index, an operator, or a name like the core SEXPs do,
instead of only a name.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Goober5000

code/parse/sexp.cpp

@@ -3970,36 +3970,38 @@ int check_sexp_syntax(int node, int desired_return_type, int recursive, int *bad
 			case OPF_GAME_SND:
 				if (node_subtype == SEXP_ATOM_NUMBER)
 				{
-					if (!gamesnd_get_by_tbl_index(atoi(CTEXT(node))).isValid())
-					{
-						return SEXP_CHECK_NUM_RANGE_INVALID;
-					}
+					int node_num = atoi(CTEXT(node));
+					if (node_num == -1)
+						break;	// explicitly allow a sound of -1, indicating either "no sound" or "default"
+					if (!gamesnd_get_by_tbl_index(node_num).isValid())
+						return SEXP_CHECK_INVALID_GAME_SND;
 				}
 				else if (node_subtype == SEXP_ATOM_STRING)
 				{
-					if (stricmp(CTEXT(node), SEXP_NONE_STRING) != 0 && !gamesnd_get_by_name(CTEXT(node)).isValid())
-					{
+					auto node_text = CTEXT(node);
+					if (stricmp(node_text, SEXP_NONE_STRING) == 0)
+						break;	// explicitly allow "no sound", although some sexps might interpret it as "default"
+					if (!gamesnd_get_by_name(node_text).isValid())
 						return SEXP_CHECK_INVALID_GAME_SND;
-					}
 				}
+				else if (node_subtype != SEXP_ATOM_LIST)
+					return SEXP_CHECK_INVALID_GAME_SND;
 				break;
 
 			case OPF_FIREBALL:
 				if (node_subtype == SEXP_ATOM_NUMBER || can_construe_as_integer(CTEXT(node)))
 				{
 					int num = atoi(CTEXT(node));
 					if (!SCP_vector_inbounds(Fireball_info, num))
-					{
-						return SEXP_CHECK_NUM_RANGE_INVALID;
-					}
+						return SEXP_CHECK_INVALID_FIREBALL;
 				}
 				else if (node_subtype == SEXP_ATOM_STRING)
 				{
 					if (fireball_info_lookup(CTEXT(node)) < 0)
-					{
 						return SEXP_CHECK_INVALID_FIREBALL;
-					}
 				}
+				else if (node_subtype != SEXP_ATOM_LIST)
+					return SEXP_CHECK_INVALID_FIREBALL;
 				break;
 
 			case OPF_SPECIES:

code/parse/sexp.h

@@ -25,6 +25,7 @@ class object;
 class waypoint;
 class p_object;
 struct ship_obj;
+class gamesnd_id;
 
 // bumped to 30 by Goober5000
 #define	OPERATOR_LENGTH	30  // if this ever exceeds TOKEN_LENGTH, let JasonH know!
@@ -1481,6 +1482,7 @@ extern int run_sexp(const char* sexpression, bool run_eval_num = false, bool *is
 extern int stuff_sexp_variable_list();
 extern int eval_sexp(int cur_node, int referenced_node = -1);
 extern int eval_num(int n, bool &is_nan, bool &is_nan_forever);
+extern gamesnd_id sexp_get_sound_index(int node);
 extern bool is_sexp_true(int cur_node, int referenced_node = -1);
 extern bool map_opf_to_opr(sexp_opf_t opf_type, sexp_opr_t &opr_type);
 const char *opr_type_name(sexp_opr_t opr_type);

code/parse/sexp/LuaSEXP.cpp

@@ -262,8 +262,9 @@ luacpp::LuaValue LuaSEXP::sexpToLua(int node, int argnum, int parent_node) const
 		return LuaValue::createValue(_action.getLuaState(), l_Weaponclass.Set(weapon_info_lookup(name)));
 	}
 	case OPF_GAME_SND: {
-		auto name = CTEXT(node);
-		return LuaValue::createValue(_action.getLuaState(), l_SoundEntry.Set(sound_entry_h(gamesnd_get_by_name(name))));
+		// handle a table index, an operator, or a name (matching what the core SEXPs accept);
+		// -1 or <none> yields an invalid handle that the script can check with :isValid()
+		return LuaValue::createValue(_action.getLuaState(), l_SoundEntry.Set(sound_entry_h(sexp_get_sound_index(node))));
 	}
 	case OPF_STRING: {
 		auto text = CTEXT(node);