Fix min parallelism when threads fail to run.

src/engine/tasks.cpp

@@ -222,8 +222,15 @@ inline executor::implementation::implementation(unsigned parallelism)
 	runners.reserve(parallelism);
 	for (; parallelism > 0; --parallelism)
 	{
-		running_count += 1;
-		runners.emplace_back([this]() { runner(); });
+		try
+		{
+			runners.emplace_back([this]() { runner(); });
+			running_count += 1;
+		}
+		catch (const std::system_error & e)
+		{
+			err_printf("Task execution error: %s");
+		}
 	}
 #endif
 }
@@ -348,7 +355,8 @@ executor::~executor() { i->stop(); }
 
 std::shared_ptr<group> executor::make(int parallelism)
 {
-	auto result = std::make_shared<group>(*this, get_parallelism(parallelism));
+	auto result = std::make_shared<group>(*this,
+		std::min(unsigned(i->runners.size()), get_parallelism(parallelism)));
 	i->push_group(result);
 	return result;
 }