Programmable pre-signals: Fix MP desync with programming new signals

If signal was never evaluated on the server, inserting instructions
would fail on the server, such that the signal programs differed
between client and server.

src/programmable_signals.cpp

@@ -530,7 +530,9 @@ SignalProgram *GetSignalProgram(SignalReference ref)
 	if (!pr) {
 		pr = new SignalProgram(ref.tile, ref.track);
 		_signal_programs[ref] = pr;
-	} else assert(pr->tile == ref.tile && pr->track == ref.track);
+	} else {
+		assert(pr->tile == ref.tile && pr->track == ref.track);
+	}
 	return pr;
 }
 
@@ -558,7 +560,8 @@ void FreeSignalPrograms()
 
 SignalState RunSignalProgram(SignalReference ref, uint num_exits, uint num_green)
 {
-	SignalProgram *program = GetSignalProgram(ref);
+	SignalProgram *program = GetExistingSignalProgram(ref);
+	if (!program) return SIGNAL_STATE_RED;
 	SignalVM vm;
 	vm.program = program;
 	vm.num_exits = num_exits;
@@ -575,7 +578,8 @@ SignalState RunSignalProgram(SignalReference ref, uint num_exits, uint num_green
 
 void RemoveProgramDependencies(SignalReference dependency_target, SignalReference signal_to_update)
 {
-	SignalProgram *prog = GetSignalProgram(signal_to_update);
+	SignalProgram *prog = GetExistingSignalProgram(signal_to_update);
+	if (!prog) return;
 	for (SignalInstruction *insn : prog->instructions) {
 		if (insn->Opcode() == PSO_IF) {
 			SignalIf* ifi = static_cast<SignalIf*>(insn);
@@ -595,7 +599,8 @@ void RemoveProgramDependencies(SignalReference dependency_target, SignalReferenc
 
 void RemoveProgramSlotDependencies(TraceRestrictSlotID slot_being_removed, SignalReference signal_to_update)
 {
-	SignalProgram *prog = GetSignalProgram(signal_to_update);
+	SignalProgram *prog = GetExistingSignalProgram(signal_to_update);
+	if (!prog) return;
 	for (SignalInstruction *insn : prog->instructions) {
 		if (insn->Opcode() == PSO_IF) {
 			SignalIf* ifi = static_cast<SignalIf*>(insn);
@@ -615,7 +620,8 @@ void RemoveProgramSlotDependencies(TraceRestrictSlotID slot_being_removed, Signa
 
 void RemoveProgramCounterDependencies(TraceRestrictCounterID ctr_being_removed, SignalReference signal_to_update)
 {
-	SignalProgram *prog = GetSignalProgram(signal_to_update);
+	SignalProgram *prog = GetExistingSignalProgram(signal_to_update);
+	if (!prog) return;
 	for (SignalInstruction *insn : prog->instructions) {
 		if (insn->Opcode() == PSO_IF) {
 			SignalIf* ifi = static_cast<SignalIf*>(insn);
@@ -669,7 +675,7 @@ CommandCost CmdInsertSignalInstruction(TileIndex tile, DoCommandFlag flags, uint
 	if (!IsTileOwner(tile, _current_company))
 		return_cmd_error(STR_ERROR_AREA_IS_OWNED_BY_ANOTHER);
 
-	SignalProgram *prog = GetExistingSignalProgram(SignalReference(tile, track));
+	SignalProgram *prog = GetSignalProgram(SignalReference(tile, track));
 	if (!prog)
 		return_cmd_error(STR_ERR_PROGSIG_NOT_THERE);
 	if (instruction_id >= prog->instructions.size())