SystemInformation: Fix unused assignment warnings

Fix interaction between local variables and conditional code in methods

 SystemInformationImplementation::GetFullyQualifiedDomainName
 SystemInformationImplementation::GetMemoryTotal
 SystemInformationImplementation::GetMemoryUsed
 SystemInformationImplementation::GetProcessId

to avoid assigning values to local variables that are never used.

While at it, fix GetMemoryUsed() to not leak the OpenProcess handle when
GetProcessMemoryInfo fails.

Change-Id: I582b70c7b4795df6b97bdeafc6328001dc5ce3ee