Initialize BaseCryptStream::nextCharBuff on construction

Otherwise if we construct a DecryptStream and next we call lookupChar it
would use un-initialized memory
diff --git a/poppler/Decrypt.cc b/poppler/Decrypt.cc
index 6d9c14f..cc343a5 100644
--- a/poppler/Decrypt.cc
+++ b/poppler/Decrypt.cc
@@ -355,6 +355,7 @@
   }
 
   charactersRead = 0;
+  nextCharBuff = EOF;
   autoDelete = true;
 }