test: fix nilptr3 check for wasm

CL 131735 only updated nilptr3.go for the adjusted nil check. Adjust
nilptr3_wasm.go as well.

Change-Id: I4a6257d32bb212666fe768dac53901ea0b051138
Reviewed-on: https://go-review.googlesource.com/133495
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/nilptr3_wasm.go

@@ -246,8 +246,8 @@ type TT struct {
 
 func f(t *TT) *byte {
 	// See issue 17242.
-	s := &t.SS  // ERROR "removed nil check"
-	return &s.x // ERROR "generated nil check"
+	s := &t.SS  // ERROR "generated nil check"
+	return &s.x // ERROR "removed nil check"
 }
 
 // make sure not to do nil check for newobject