Fix crash in best_group() (#1891)

This pull request fixes a rare crashing bug on Windows inside our NUMA code, first
reported by Dann Corbit in the following forum thread (thanks!):
https://groups.google.com/forum/?fromgroups=#!topic/fishcooking/gA6aoMEuOwg

The fix is to not use structure member beyond known size when iterating through
'SYSTEM_LOGICAL_PROCESSOR_INFORMATION_EX' structure. We note that the Microsoft
API is guaranteed to provide us at least one element upon successful, and no
element in the structure can have a zero size.

No functional change.

src/misc.cpp

@@ -257,7 +257,7 @@ int best_group(size_t idx) {
       return -1;
   }
 
-  while (ptr->Size > 0 && byteOffset + ptr->Size <= returnLength)
+  while (byteOffset < returnLength)
   {
       if (ptr->Relationship == RelationNumaNode)
           nodes++;
@@ -268,6 +268,7 @@ int best_group(size_t idx) {
           threads += (ptr->Processor.Flags == LTP_PC_SMT) ? 2 : 1;
       }
 
+      assert(ptr->Size);
       byteOffset += ptr->Size;
       ptr = (SYSTEM_LOGICAL_PROCESSOR_INFORMATION_EX*)(((char*)ptr) + ptr->Size);
   }