API:

- freeing strings returned by RNA struct functions in RNA_parameter_list_free

Unit tests:
- check that BKE_export_image actually creates a file. This test is becoming dangerous: it creates and deletes files under /tmp.

Having written this complicated test function I now realize it's much easier to write tests in a scripted language, which gives more 
freedom in expressions and need not be compiled.

source/blender/python/intern/bpy_rna.c

@@ -1844,7 +1844,8 @@ PyObject *pyrna_param_to_py(PointerRNA *ptr, PropertyRNA *prop, void *data)
 			break;
 		case PROP_STRING:
 		{
-			ret = PyUnicode_FromString( *(char**)data );
+			char *ptr = *(char**)data;
+			ret = ptr ? PyUnicode_FromString( ptr ) : Py_None;
 			break;
 		}
 		case PROP_ENUM: