io: Coverity fixes

Coverity fixes for the iomgr & pnpmgr :

• CID 514673: Dereference null return value (ntoskrnl/io/iomgr/device.c) --> add a check on Irp as it's done in Phase 0.

• CID 716581: Resource leak (ntoskrnl/io/iomgr/driver.c) --> Free memory.

• CID 514576: Unchecked return value (ntoskrnl/io/iomgr/ioevent.c) --> ObReferenceObjectByHandle can return NULL : handle this case.

• In ntoskrnl/io/iomgr/iofunc.c : Initialize NormalContext variable.

• CID 716713: Destination buffer too small (ntoskrnl/io/iomgr/iorsrce.c) --> Use RtlInitUnicodeString instead, and the corresponding buffer will be the stack memory place where the strings are stored.

• CID 716269 & 716203: Out-of-bounds access (ntoskrnl/io/iomgr/ramdisk.c) : same problem as in init.c --> sizeof(SourceString)/sizeof(WCHAR)

• CID 514672: Dereference null return value (ntoskrnl/io/iomgr/volume.c) --> as in device.c

• CID 716591: Resource leak (ntoskrnl/io/pnpmgr/pnpnotify.c) --> ExFreePool => ExFreePoolWithTag.

• CID 514643: Missing break in switch (ntoskrnl/io/pnpmgr/pnpnotify.c) --> add it (plus code formatting).