ob: po: Coverity defects fixes

Coverity fixes :

• CID 716763: Unitialized scalar variable (ntoskrnl/ob/oblife.c)
• CID 716601: Resource leak (ntoskrnl/ob/oblink.c) --> SymbolicLink (allocated via ObCreateObject) isn't freed if failing --> Call ObDereferenceObject.
• CID 716602: Resource leak (ntoskrnl/ob/obsdcach.c) --> call ExFreePoolWithTag (tag --> that of ObpCreateCacheEntry)
• CID 701419 & 420 & 421: Unitialized pointer read (ntoskrnl/ob/obsecure.c)
• CID 514680 & 671: Dereference null return value (ntoskrnl/po/power.c)