Fix a small memleak in write_to_buffer() (#497)

Author: kleisauke

Diff for: pyvips/vimage.py

@@ -849,6 +849,7 @@ def write_to_buffer(self, format_string, **kwargs):
         format_string = _to_bytes(format_string)
 
         filename = vips_lib.vips_filename_get_filename(format_string)
+        filename = ffi.gc(filename, glib_lib.g_free)
 
         pointer = vips_lib.vips_filename_get_options(format_string)
         options = _to_string_copy(pointer)