Fix DoReadMemory for Windows mini dumps.

[amccarth]

Summary

The operands of a subtraction were backwards. Thus the offset into the range for the memory we desired was wrong. This corrects the subtraction and adds some assertions for sanity.

This was the cause of the inability to get a full backtrace or to evaluate expressions involving local variables.

source/Plugins/Process/Windows/MiniDump/ProcessWinMiniDump.cpp

 #include "lldb/Core/PluginManager.h"
 #include "lldb/Core/Section.h"
 #include "lldb/Core/State.h"
 #include "lldb/Target/DynamicLoader.h"
 #include "lldb/Target/MemoryRegionInfo.h"
 #include "lldb/Target/StopInfo.h"
 #include "lldb/Target/Target.h"
 #include "lldb/Target/UnixSignals.h"
+#include "lldb/Utility/LLDBAssert.h"
 #include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include "ExceptionRecord.h"
 #include "ThreadWinMiniDump.h"
 
 using namespace lldb_private;
ProcessWinMiniDump::DoReadMemory(lldb::addr_t addr, void *buf, size_t size, Error &error)
     if (!FindMemoryRange(addr, &range))
     {
         return 0;
     }
 
     // There's at least some overlap between the beginning of the desired range
     // (addr) and the current range.  Figure out where the overlap begins and
     // how much overlap there is, then copy it to the destination buffer.
-    const size_t offset = range.start - addr;
+    lldbassert(range.start <= addr);
+    const size_t offset = addr - range.start;
+    lldbassert(offset < range.size);
     const size_t overlap = std::min(size, range.size - offset);
     std::memcpy(buf, range.ptr + offset, overlap);
     return overlap;
 }
 
 Error
 ProcessWinMiniDump::GetMemoryRegionInfo(lldb::addr_t load_addr, lldb_private::MemoryRegionInfo &info)
 {