Add an assert to check copy_to_buffer's precondition.

llvm-svn: 78926

lib/Support/raw_ostream.cpp

@@ -222,6 +222,8 @@ raw_ostream &raw_ostream::write(const char *Ptr, size_t Size) {
 }
 
 void raw_ostream::copy_to_buffer(const char *Ptr, size_t Size) {
+  assert(Size <= OutBufEnd - OutBufCur && "Buffer overrun!");
+
   // Handle short strings specially, memcpy isn't very good at very short
   // strings.
   switch (Size) {