Make llGetLinkPrimitiveParams() abort and return existing list of params when it encounters an invalid link number, rather than throwing an exception

Addresses http://opensimulator.org/mantis/view.php?id=6768
Thanks to talun for the patch on that commit - in the end I took a different approach that also deals with invalid PRIM_LINK_TARGET
However, not yet generating the same warning on invalid PRIM_LINK_TARGET as seen on LL grid
This commit also adds regression tests for some cases of llGetLinkPrimitiveParams()