MSVC build fix following r211749

Avoid strndup()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211752 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IRReader/IRReader.cpp

@@ -110,7 +110,8 @@ LLVMBool LLVMParseIRInContext(LLVMContextRef ContextRef,
     if (OutMessage) {
       string_ostream os;
       Diag.print(nullptr, os, false);
-      *OutMessage = strndup(os.str().data(), os.str().size());
+      os << '\0';
+      *OutMessage = strdup(os.str().data());
     }
     return 1;
   }