Fixed a looming memory corruption bug.

malloc does not initialize the memory to zero, so

sass_context *ctx = sass_new_context()

will create a context with a random value in ctx->output_string.

If sass_free_context(ctx) was called immediately thereafter,
ctx->output_string would have a random value and the result of the
free(ctx->output_string) would be undefined.

In the worst case, this corrupts the heap and the process dies much much later.

Also, free isn't delete and mustn't be called with a NULL pointer.

sass_interface.cpp

@@ -13,26 +13,28 @@ extern "C" {
 
   using namespace std;
   sass_context* sass_new_context()
-    { return (sass_context*) malloc(sizeof(sass_context)); }
+    { return (sass_context*) calloc(1, sizeof(sass_context)); }
   
   void sass_free_context(sass_context* ctx)
   { 
-    free(ctx->output_string);
+    if (ctx->output_string)
+      free(ctx->output_string);
     free(ctx);
   }
 
   sass_file_context* sass_new_file_context()
-    { return (sass_file_context*) malloc(sizeof(sass_file_context)); }
+    { return (sass_file_context*) calloc(1, sizeof(sass_file_context)); }
   
   void sass_free_file_context(sass_file_context* ctx)
   { 
-    free(ctx->output_string);
+    if (ctx->output_string)
+      free(ctx->output_string);
     free(ctx);
   }
   
   sass_folder_context* sass_new_folder_context()
-    { return (sass_folder_context*) malloc(sizeof(sass_folder_context)); }
-  
+    { return (sass_folder_context*) calloc(1, sizeof(sass_folder_context)); }
+
   static char* process_document(Sass::Document& doc, int style)
   {
     using namespace Sass;