[llvm-pdbdump] Try to appease the ASan bot

We didn't check that the file was large enough to hold a super block.

llvm-svn: 267965
diff --git a/llvm/lib/DebugInfo/PDB/Raw/PDBFile.cpp b/llvm/lib/DebugInfo/PDB/Raw/PDBFile.cpp
index be1368e..0adabfe 100644
--- a/llvm/lib/DebugInfo/PDB/Raw/PDBFile.cpp
+++ b/llvm/lib/DebugInfo/PDB/Raw/PDBFile.cpp
@@ -122,6 +122,11 @@
   Context->SB =
       reinterpret_cast<const SuperBlock *>(BufferRef.getBufferStart());
   const SuperBlock *SB = Context->SB;
+
+  // Make sure the file is sufficiently large to hold a super block.
+  if (BufferRef.getBufferSize() < sizeof(SuperBlock))
+    return std::make_error_code(std::errc::illegal_byte_sequence);
+
   // Check the magic bytes.
   if (memcmp(SB->MagicBytes, Magic, sizeof(Magic)) != 0)
     return std::make_error_code(std::errc::illegal_byte_sequence);