fixes nil panic on close (#1286)

The last PR over-solved the WASI close problem as it special cased both
"_start", used by WASI, and the start section (wasm level) which isn't
used by WASI. In the latter case, we ended up returning nil for both the
module *and* the error result. We should never do that.. If we coerce
exit error zero to nil, we have to return a non-nil module, even if it
is unusable as otherwise code like `defer mod.Close(ctx)` will panic.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

runtime.go

@@ -308,11 +308,6 @@ func (r *runtime) InstantiateModule(
 		if code.closeWithModule {
 			_ = code.Close(ctx) // don't overwrite the error
 		}
-		if se, ok := err.(*sys.ExitError); ok {
-			if se.ExitCode() == 0 { // Don't err on success.
-				err = nil
-			}
-		}
 		return
 	}