src: fix invalid windowBits=8 gzip segfault

`{ windowBits: 8 }` is legal for deflate streams but not gzip streams.
Fix a nullptr dereference when formatting the error message.

Bug introduced in commit c34eae5f88 ("zlib: refactor zlib internals")
from September 2018.

PR-URL: https://github.com/nodejs/node/pull/33045
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: David Carlier <devnexen@gmail.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/node_zlib.cc b/src/node_zlib.cc
index eacd710143a..83698bd5192 100644
--- a/src/node_zlib.cc
+++ b/src/node_zlib.cc
@@ -111,7 +111,12 @@ enum node_zlib_mode {
 
 struct CompressionError {
   CompressionError(const char* message, const char* code, int err)
-    : message(message), code(code), err(err) {}
+      : message(message),
+        code(code),
+        err(err) {
+    CHECK_NOT_NULL(message);
+  }
+
   CompressionError() = default;
 
   const char* message = nullptr;
@@ -997,7 +1002,7 @@ CompressionError ZlibContext::Init(
   if (err_ != Z_OK) {
     dictionary_.clear();
     mode_ = NONE;
-    return ErrorForMessage(nullptr);
+    return ErrorForMessage("zlib error");
   }
 
   return SetDictionary();