This patch fixes the segmentation fault discovered in _accumpart() due to incorrect usage of _cpyif() by adding an offset parameter to _cpyif() so that the intent of setting buf to NULL can be cleanly transmitted to the test in _cpyif(). It was decided that this is less brittle than simply testing buf in one case because the offset parameter cannot be forgotten without causing a compile error, whereas the test can be (and was) easily forgotten.
This patch fixes the segmentation fault discovered in `_accumpart()` due to incorrect usage of `_cpyif()` by adding an offset parameter to `_cpyif()` so that the intent of setting `buf` to `NULL` can be cleanly transmitted to the test in `_cpyif()`. It was decided that this is less brittle than simply testing `buf` in one case because the offset parameter cannot be forgotten without causing a compile error, whereas the test can be (and was) easily forgotten.