Gzip: fixed zlib memLevel adjusting.

An incorrect memLevel (lower than 1) might be passed to deflateInit2() if the
"gzip_hash" directive is set to a value less than the value of "gzip_window"
directive. This resulted in "deflateInit2() failed: -2" alert and an empty
reply.

src/http/modules/ngx_http_gzip_filter_module.c

@@ -498,6 +498,10 @@ ngx_http_gzip_filter_memory(ngx_http_request_t *r, ngx_http_gzip_ctx_t *ctx)
             wbits--;
             memlevel--;
         }
+
+        if (memlevel < 1) {
+            memlevel = 1;
+        }
     }
 
     ctx->wbits = wbits;