Possible fix for XP SP0 System process exploit crash

Exploitation of the System process on Windows XP SP0 resulted in crashes
when metepreter was enumerating network interfaces. It appears that the
System process isn't able to read from the address that contains prefixes
(stored in pPrefix in interfaces.c). In other cases, such as exploitation
of svchost.exe or via an msfpayload-generated exe, there was no such
crash. This is not an issue on later versions of Windows.

This crash happens in the current "production" version of Meterpreter that
was deployed with MSF v4.7.

The MS08-067 exploit targets the System process, and hence crashes.
The MS03-26 exploit targets svchost, and hence does not crash.

Checking the protection of the memory in each of those cases showed that
the MEM_COMMIT was set when not the System process, and was MEM_RESERVED
when it was. Sample runs can be seen here: http://pastebin.com/2WHqJ90A

This commit is an attempt to avoid this crash down the track, and it just
does a simple check to see if querying the area of memory for the current
process shows a state of MEM_COMMIT. If so, it carries on as per normal,
if not it uses the other legacy code path to traverse the network
interfaces.

With this "fix" in place I have not seen a crash at all on Windows XP,
Windows 7, Windows 8 and Windows 2012.