[Bitcode] AtEndOfStream should only check against the size if it's known.

This avoids an issue where AtEndOfStream mistakenly returns true at the /start/ of
a stream.

(In the rare case that the size is known and actually 0, the slow path will still
handle it correctly.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221840 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitstreamReader.h

@@ -227,7 +227,7 @@ public:
   bool AtEndOfStream() {
     if (BitsInCurWord != 0)
       return false;
-    if (Size == NextChar)
+    if (Size != 0 && Size == NextChar)
       return true;
     fillCurWord();
     return BitsInCurWord == 0;