Zero-initialize the root pointer.

A casualty of one too many prospective refactors, and of
zero-initialized stacks.

Swift SVN r32456

include/swift/Basic/PrefixMap.h

@@ -341,7 +341,7 @@ private:
     return root;
   }
 
-  Node *Root;
+  Node *Root = nullptr;
 
 public:
   PrefixMap() {}