[GR-67938] Fix wasm launcher --entry-point option.

PullRequest: graal/21580

Author: woess

wasm/src/org.graalvm.wasm.launcher/src/org/graalvm/wasm/launcher/WasmLauncher.java

@@ -169,10 +169,10 @@ private int execute(Context.Builder contextBuilder) {
     }
 
     private Value detectEntryPoint(Value mainModule) {
+        Value exports = mainModule.getMember("exports");
         if (customEntryPoint != null) {
-            return mainModule.getMember(customEntryPoint);
+            return exports.getMember(customEntryPoint);
         }
-        Value exports = mainModule.getMember("exports");
         Value candidate = exports.getMember("_start");
         if (candidate == null) {
             candidate = exports.getMember("_main");