Initialize rollingPruningNumParts in init_plist

Initialize rollingPruningNumParts in init_plist

Previously, rollingPruningNumParts and rollingPruningPart were not
initialized, which, in the case of an empty domain, caused access
to uninitialized memory. No particular harm was being done, but it
could trigger an assertion in gpu_launch_kernel_pruneonly during
some of the regression tests.

The whole code in the preamble of gpu_launch_kernel_pruneonly is
probably due for refactoring, but since we initialize all other
plist's fields in init_plist, we better initialize these two as well.