Fix source_c_str handling (don't copy since it is char*)

Addresses sass#869

This is a breaking change since implementers need to
make a copy of the string if they are not able to pass
the complete memory ownership to libsass.

context.cpp

@@ -333,9 +333,10 @@ namespace Sass {
     if(is_indented_syntax_src) {
       char * contents = sass2scss(source_c_str, SASS2SCSS_PRETTIFY_1 | SASS2SCSS_KEEP_COMMENT);
       add_source(input_path, input_path, contents);
+      delete [] source_c_str;
       return parse_file();
     }
-    add_source(input_path, input_path, sass_strdup(source_c_str));
+    add_source(input_path, input_path, source_c_str);
     return parse_file();
   }
 

sass_context.cpp

@@ -760,7 +760,7 @@ return 0;
 
     struct string_list* plugin_path = (struct string_list*) calloc(1, sizeof(struct string_list));
     if (plugin_path == 0) return;
-    plugin_path->string = path ? copy_c_str(path) : 0;
+    plugin_path->string = path ? sass_strdup(path) : 0;
     struct string_list* last = options->plugin_paths;
     if (!options->plugin_paths) {
       options->plugin_paths = plugin_path;