Windows: Fix DeRegisterEHFrames from erasing a possibly invalid iterator.

DeRegisterEHFrames can be called without a a prior call to RegisterEHFrames.
This can easily be detected so just do nothing when that occurs.

Author: marsupial

lib/Utils/PlatformWin.cpp

@@ -788,11 +788,17 @@ void RegisterEHFrames(uintptr_t ImgBs, const EHFrameInfos& Frames, bool Block) {
 void DeRegisterEHFrames(uintptr_t ImgBase, const EHFrameInfos& Frames) {
   if (Frames.empty())
     return;
-  assert(getImageBaseMap().find(ImgBase) != getImageBaseMap().end());
 
-  // Remove the ImageBase from lookup
+  // There is a chance that DeRegisterEHFrames will without a prior call to
+  // RegisterEHFrames.  Rather than tracking such cases and adding complexity
+  // just ignore the requests if ImgBase was never registered.
   ImageBaseMap& Unwind = getImageBaseMap();
-  Unwind.erase(Unwind.find(ImgBase));
+  auto Itr = Unwind.find(ImgBase);
+  if (Itr == Unwind.end())
+    return;
+
+  // Remove the ImageBase from lookup
+  Unwind.erase(Itr);
 
   // Unregister all the PRUNTIME_FUNCTIONs
   for (auto&& Frame : Frames)