Refs #4472 fix MemoryStats returning negative available memory(!)

which overflowed the unsigned int giving billions of GB available

Code/Mantid/Framework/Kernel/src/Memory.cpp

@@ -212,7 +212,7 @@ void MemoryStats::process_mem_system(size_t & sys_avail, size_t & sys_total)
     sys_avail = totPages / 1024 * pageSize;
   }
   // Can get the info on the memory that we've already obtained but aren't using right now
-  const int unusedReserved = mallinfo().fordblks/1024;
+  int unusedReserved = mallinfo().fordblks/1024;
   // unusedReserved can sometimes be negative, which wen added to a low sys_avail will overflow the unsigned int.
   if (unusedReserved < 0) unusedReserved = 0;
   // g_log.debug() << "Linux - Adding reserved but unused memory of " << unusedReserved << " KB\n";