runtime: fix nil pointer in TestGoroutineParallelism2 when offline

runtime: fix nil pointer in TestGoroutineParallelism2 when offline

Previously, the test would crash when running on a computer without an
internet connection, e.g. in airplane mode (stack trace below).

The bug was that the condition was inverted. The code tried to close
the listener if `err != nil` (that is, if net.Listen() failed). But if
Listen() failed then there is no listener to close! The listener
should only be closed if Listen() succeeded.

Here is the stack trace from `go test runtime` when offline:

```
--- FAIL: TestGoroutineParallelism2 (0.16s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7bdaa1]

goroutine 3858 gp=0xc000185180 m=5 mp=0xc000100008 [running]:
panic({0x854960?, 0xbf70b0?})
	<go>/src/runtime/panic.go:778 +0x168 fp=0xc0000afad8 sp=0xc0000afa28 pc=0x441bc8
testing.tRunner.func1.2({0x854960, 0xbf70b0})
	<go>/src/testing/testing.go:1632 +0x230 fp=0xc0000afb88 sp=0xc0000afad8 pc=0x524090
testing.tRunner.func1()
	<go>/src/testing/testing.go:1635 +0x35e fp=0xc0000afd18 sp=0xc0000afb88 pc=0x523a7e
panic({0x854960?, 0xbf70b0?})
	<go>/src/runtime/panic.go:759 +0x132 fp=0xc0000afdc8 sp=0xc0000afd18 pc=0x441b92
runtime.panicmem(...)
	<go>/src/runtime/panic.go:261
runtime.sigpanic()
	<go>/src/runtime/signal_unix.go:900 +0x359 fp=0xc0000afe28 sp=0xc0000afdc8 pc=0x483c79
runtime_test.testGoroutineParallelism2(0x522e13?, 0x0, 0x1)
	<go>/src/runtime/proc_test.go:204 +0x221 fp=0xc0000aff50 sp=0xc0000afe28 pc=0x7bdaa1
runtime_test.TestGoroutineParallelism2(0xc000221520)
	<go>/src/runtime/proc_test.go:151 +0x30 fp=0xc0000aff70 sp=0xc0000aff50 pc=0x7bd850
testing.tRunner(0xc000221520, 0x8fed88)
	<go>/src/testing/testing.go:1690 +0xf4 fp=0xc0000affc0 sp=0xc0000aff70 pc=0x523674
testing.(*T).Run.gowrap1()
	<go>/src/testing/testing.go:1743 +0x25 fp=0xc0000affe0 sp=0xc0000affc0 pc=0x524665
runtime.goexit({})
	<go>/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc0000affe8 sp=0xc0000affe0 pc=0x487a41
created by testing.(*T).Run in goroutine 1
	<go>/src/testing/testing.go:1743 +0x390
```

Change-Id: I48983fe21b3360ea9d0182c4a3b509801257027b
Reviewed-on: https://go-review.googlesource.com/c/go/+/584436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Keith Randall <khr@google.com>