conf: fix snd_func_imul accumulator initialization

alsa-project/alsa-lib pull request #511 was opened from orange-dot:

snd_func_iops() initialized result to 0 for both iadd and imul. For
imul, every multiplication step therefore multiplied by 0, so @func imul
always evaluated to 0.

Initialize the accumulator to 1 for imul, while preserving the existing
0 result for an empty integers list. Add regression coverage for iadd and
imul evaluation.

Fixes: https://github.com/alsa-project/alsa-lib/issues/456

Request URL   : https://github.com/alsa-project/alsa-lib/pull/511
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/511.patch
Repository URL: https://github.com/alsa-project/alsa-lib