pkg/ipc: fix potential nil deref
pkg/ipc: fix potential nil deref

We've got the following crash:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x7d30ba]
goroutine 75 [running]:
github.com/google/syzkaller/pkg/ipc.(*command).exec(0xc000c68cb0, 0xc0001a4d20, 0xc00147a000, 0x1f28, 0x200000, 0x0, 0x0, 0xc00170a000, 0x1000000, 0x1000000, ...)
        /syzkaller/gopath/src/github.com/google/syzkaller/pkg/ipc/ipc.go:783 +0x7ca
github.com/google/syzkaller/pkg/ipc.(*Env).Exec(0xc0002de240, 0xc0001a4d20, 0xc016c2f440, 0x11, 0xc00019c7e0, 0x40, 0x1, 0xc016b6f590, 0x30, 0xc01615cc40)
        /syzkaller/gopath/src/github.com/google/syzkaller/pkg/ipc/ipc.go:280 +0x104
main.(*Proc).executeRaw(0xc0012a45c0, 0xc0001a4d20, 0xc016c2f440, 0x1, 0x0)
        /syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:292 +0x1f3
main.(*Proc).execute(0xc0012a45c0, 0xc0001a4d20, 0xc016c2f440, 0x0, 0x1, 0x1)
        /syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:255 +0x6a
main.(*Proc).loop(0xc0012a45c0)
        /syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:101 +0x4df
created by main.main
        /syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:259 +0x1153

err can be nil even if hang=true in case of a narrow race:
we decided to kill the process, but it finished successfully meanwhile.

google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller