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.