Fix GH-16009: Segmentation fault with frameless functions and undefined CVs

The frameless function handlers do not update the op variables when
handling the result is undefined. In this

Fix GH-16009: Segmentation fault with frameless functions and undefined CVs

The frameless function handlers do not update the op variables when
handling the result is undefined. In this case this causes propagating
an UNDEF value into a temporary, which results in an extra undefined
variable warning for a temporary in this case.

The original issue also reports a crash in some cases, which is also
fixed by this patch.

Closes GH-16012.

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