net/http: fix nil panic in test

Observed on a builder in an unrelated CL.
https://logs.chromium.org/logs/golang/buildbucket/cr-buildbucket/8728107031663629713/+/u/step/11/log/2

goroutine 27937 gp=0xc00000f6c0 m=20 mp=0xc000085008 [running]:
panic({0x560ac0?, 0xa1f400?})
	C:/b/s/w/ir/x/w/goroot/src/runtime/panic.go:806 +0x168 fp=0xc00043fac8 sp=0xc00043fa18 pc=0xa5f88
testing.tRunner.func1.2({0x560ac0, 0xa1f400})
	C:/b/s/w/ir/x/w/goroot/src/testing/testing.go:1734 +0x219 fp=0xc00043fb78 sp=0xc00043fac8 pc=0x1537f9
testing.tRunner.func1()
	C:/b/s/w/ir/x/w/goroot/src/testing/testing.go:1737 +0x359 fp=0xc00043fce0 sp=0xc00043fb78 pc=0x153259
panic({0x560ac0?, 0xa1f400?})
	C:/b/s/w/ir/x/w/goroot/src/runtime/panic.go:787 +0x132 fp=0xc00043fd90 sp=0xc00043fce0 pc=0xa5f52
runtime.panicmem(...)
	C:/b/s/w/ir/x/w/goroot/src/runtime/panic.go:262
runtime.sigpanic()
	C:/b/s/w/ir/x/w/goroot/src/runtime/signal_windows.go:401 +0x198 fp=0xc00043fdd8 sp=0xc00043fd90 pc=0x87938
net/http_test.testTransportIdleConnTimeout.func3(...)
	C:/b/s/w/ir/x/w/goroot/src/net/http/transport_test.go:5503
net/http_test.testTransportIdleConnTimeout(0xc000e83340, {0x5ec863, 0x2})
	C:/b/s/w/ir/x/w/goroot/src/net/http/transport_test.go:5522 +0x4c1 fp=0xc00043ff20 sp=0xc00043fdd8 pc=0x47a841
net/http_test.run[...].func1()
	C:/b/s/w/ir/x/w/goroot/src/net/http/clientserver_test.go:93 +0xfe fp=0xc00043ff70 sp=0xc00043ff20 pc=0x49a21e
testing.tRunner(0xc000e83340, 0xc0004687b0)
	C:/b/s/w/ir/x/w/goroot/src/testing/testing.go:1792 +0xcb fp=0xc00043ffc0 sp=0xc00043ff70 pc=0x152e4b
testing.(*T).Run.gowrap1()
	C:/b/s/w/ir/x/w/goroot/src/testing/testing.go:1851 +0x25 fp=0xc00043ffe0 sp=0xc00043ffc0 pc=0x153e65
runtime.goexit({})
	C:/b/s/w/ir/x/w/goroot/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc00043ffe8 sp=0xc00043ffe0 pc=0xadfe1
created by testing.(*T).Run in goroutine 27899
	C:/b/s/w/ir/x/w/goroot/src/testing/testing.go:1851 +0x3f6


Change-Id: I3f8bd7c7863b5031ff43d4837dfe11d26ac75652
Reviewed-on: https://go-review.googlesource.com/c/go/+/637897
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/transport_test.go

@@ -5500,7 +5500,9 @@ timeoutLoop:
 					return false
 				}
 			}
-			res.Body.Close()
+			if err == nil {
+				res.Body.Close()
+			}
 			conns := idleConns()
 			if len(conns) != 1 {
 				if len(conns) == 0 {