Code: Sends copy of string to upstream.

LibSass changed this behavior recently that it will *own* the memory we
are passing to it. this delta makes changes to `MarshalString()` function
so it returns a copy of `char*` after freeing the converted `char*` from
`String^`.

Then it removes the unrequired `FreeString()` calls in `finally` block to
avoid "double-freeing".

Fixes #28

Author: am11

libsass/SassInterface.cpp

@@ -67,8 +67,6 @@ namespace LibSassNet
         finally
         {
             // Free resources
-            FreeString(includePaths);
-            FreeString(sourceString);
             sass_delete_data_context(ctx);
         }
     }
@@ -127,9 +125,6 @@ namespace LibSassNet
         finally
         {
             // Free resources
-            FreeString(includePaths);
-            FreeString(inputPath);
-            FreeString(mapFile);
             sass_delete_file_context(ctx);
         }
     }
@@ -156,6 +151,7 @@ namespace LibSassNet
         }
         finally
         {
+            // Upstream will not free the memory in case of sass2scss
             FreeString(sourceText);
         }
     }

libsass/StringToANSI.cpp

@@ -19,6 +19,7 @@
 //SOFTWARE.
 
 #using <System.dll>
+#include <string>
 #include "StringToANSI.hpp"
 
 using namespace System;
@@ -27,8 +28,18 @@ using namespace System::Runtime::InteropServices;
 namespace LibSassNet
 {
 	char* MarshalString(String^ s)
-	{
-        return (char*) ((Marshal::StringToCoTaskMemAnsi(s)).ToPointer());
+    {
+        if (!s) {
+            return nullptr;
+        }
+
+        char* original_str = (char*)(Marshal::StringToCoTaskMemAnsi(s)).ToPointer();
+        char* target_str = (char*)malloc(strlen(original_str) + 1);
+        strcpy(target_str, original_str);
+
+        FreeString(original_str);
+
+        return target_str;
 	}
 
 	void FreeString(const char* p)