zlib: Fix handling of gzip magic bytes mid-file

Only treat the gzip magic bytes, when encountered within the file
after reading a single block, as the start of a new member when
the previous member has ended.

Fixes: nodejs#5852

src/node_zlib.cc

@@ -258,7 +258,8 @@ class ZCtx : public AsyncWrap {
           }
         }
         while (ctx->strm_.avail_in >= GZIP_MIN_HEADER_SIZE &&
-               ctx->mode_ == GUNZIP) {
+               ctx->mode_ == GUNZIP &&
+               ctx->err_ == Z_STREAM_END) {
           // Bytes remain in input buffer. Perhaps this is another compressed
           // member in the same archive, or just trailing garbage.
           // Check the header to find out.