Fix memory leaks in localisation

Assigning a pointer to std::string appears to only perform a copy and
does not transfer ownership of the pointer, thus the allocated memory is
will never be freed.

Implement a method to construct an std::string from a StringBuilder to
avoid memory leaks when retreiving the contents of a StringBuilder and
storing it in a std::string.

src/openrct2/core/StringBuilder.hpp

@@ -10,6 +10,7 @@
 #pragma once
 
 #include <algorithm>
+#include <string>
 #include "../common.h"
 
 #include "Math.hpp"
@@ -132,6 +133,20 @@ public:
         return result;
     }
 
+    /**
+     * Returns the current string buffer as a standard string.
+     */
+    std::string GetStdString() const
+    {
+        std::string result;
+        result.reserve(_length);
+        for (std::size_t i = 0U; i < _length; i++)
+        {
+            result.push_back(_buffer[i]);
+        }
+        return result;
+    }
+
     /**
      * Gets the current state of the StringBuilder. Warning: this represents the StringBuilder's current working buffer and will
      * be deallocated when the StringBuilder is destructed.

src/openrct2/localisation/LanguagePack.cpp

@@ -389,7 +389,7 @@ private:
             }
             if (sb.GetLength() == 8)
             {
-                _currentGroup = sb.GetString();
+                _currentGroup = sb.GetStdString();
                 _currentObjectOverride = GetObjectOverride(_currentGroup);
                 _currentScenarioOverride = nullptr;
                 if (_currentObjectOverride == nullptr)
@@ -432,7 +432,7 @@ private:
 
         if (closedCorrectly)
         {
-            _currentGroup = sb.GetString();
+            _currentGroup = sb.GetStdString();
             _currentObjectOverride = nullptr;
             _currentScenarioOverride = GetScenarioOverride(_currentGroup);
             if (_currentScenarioOverride == nullptr)