While investigating a tiller crash on v2.10.0 (see recent comments in #3125), I pulled down the code
and wrote a test replicating the crash I was experiencing. I then
realized that the crash had been fixed, and was able to locate the fix
in #4630 after running a quck bisect.

Since there don't appear to be any tests that cover this crash, and I
had written one myself, I figured I might as well put up a PR for it.

Here's what the test failure on v2.10.0 looks like:
```
-- FAIL: TestUpdateReleasePendingInstall_Force (0.00s)
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=0x50 pc=0x1d128d8]

goroutine 235 [running]:
testing.tRunner.func1(0xc420493c20)
        /usr/local/Cellar/go/1.10/libexec/src/testing/testing.go:742 +0x29d
panic(0x1eb8d80, 0x2a12db0)
        /usr/local/Cellar/go/1.10/libexec/src/runtime/panic.go:505 +0x229
k8s.io/helm/pkg/tiller.(*ReleaseServer).performUpdateForce(0xc4208210b0, 0xc4202c6dc0, 0x0, 0x0, 0x2174220)
        /Users/mattrasmus/go/src/k8s.io/helm/pkg/tiller/release_update.go:166 +0x188
k8s.io/helm/pkg/tiller.(*ReleaseServer).UpdateRelease(0xc4208210b0, 0x2191780, 0xc420820f30, 0xc4202c6dc0, 0x29aeb90, 0x38, 0x2d2)
        /Users/mattrasmus/go/src/k8s.io/helm/pkg/tiller/release_update.go:43 +0x245
k8s.io/helm/pkg/tiller.TestUpdateReleasePendingInstall_Force(0xc420493c20)
        /Users/mattrasmus/go/src/k8s.io/helm/pkg/tiller/release_update_test.go:549 +0x120
testing.tRunner(0xc420493c20, 0x20e5c70)
        /usr/local/Cellar/go/1.10/libexec/src/testing/testing.go:777 +0xd0
created by testing.(*T).Run
        /usr/local/Cellar/go/1.10/libexec/src/testing/testing.go:824 +0x2e0
FAIL    k8s.io/helm/pkg/tiller  0.118s
```
Signed-off-by: Matt Rasmus <mrasmus@betterworks.com>