unref the image *after* we used it

Otherwise we manipulate potentially deallocated memory.
On some distributions things go fine for some reason, but
on others the memory actually gets deallocated and garbage is used,
leading to a segmentation fault in vips_image_guess_interpretation

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

vips.go

@@ -388,8 +388,9 @@ func vipsInterpretationBuffer(buf []byte) (Interpretation, error) {
 	if err != nil {
 		return InterpretationError, err
 	}
+	interp := vipsInterpretation(image)
 	C.g_object_unref(C.gpointer(image))
-	return vipsInterpretation(image), nil
+	return interp, nil
 }
 
 func vipsInterpretation(image *C.VipsImage) Interpretation {