src: fix use-after-return in zlib bindings

Pointed out by Coverity.  Introduced in commit 5b8e1da from September
2011 ("Initial pass at zlib bindings".)

The asynchronous version of Write() used a pointer to a stack-allocated
buffer on flush.  A mitigating factor is that zlib does not dereference
the pointer for zero-sized writes but it's still technically UB.

PR-URL: #7374
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

src/node_zlib.cc

@@ -145,8 +145,7 @@ class ZCtx : public AsyncWrap {
 
     if (args[1]->IsNull()) {
       // just a flush
-      Bytef nada[1] = { 0 };
-      in = nada;
+      in = nullptr;
       in_len = 0;
       in_off = 0;
     } else {