[AST] Fix a crash on invalid.

Problem was that we were appending to the source location info buffer in the
copy assignment operator (instead of overwriting).

rdar://42746401

llvm-svn: 341869
diff --git a/clang/lib/AST/NestedNameSpecifier.cpp b/clang/lib/AST/NestedNameSpecifier.cpp
index a514d57..df23972 100644
--- a/clang/lib/AST/NestedNameSpecifier.cpp
+++ b/clang/lib/AST/NestedNameSpecifier.cpp
@@ -547,6 +547,7 @@
   }
 
   // Deep copy.
+  BufferSize = 0;
   Append(Other.Buffer, Other.Buffer + Other.BufferSize, Buffer, BufferSize,
          BufferCapacity);
   return *this;