ZLib should check for _compressFile rather than _main to see if the module has been instantiated

Since _main isn't exported by the module we end up recreating the instance/module on each iteration.

Author: kmiller68

wasm/zlib/benchmark.js

@@ -4,7 +4,7 @@
 
 class Benchmark {
   async runIteration() {
-    if (!Module._main)
+    if (!Module._compressFile)
       await setupModule(Module);
 
     Module.FS.writeFile('input', Module.wasmBinary);