cmd/debug: execute p.Signal only when p is not nil (#4271)

* cmd/debug: execute p.Signal when p is not nil

https://github.com/tendermint/tendermint/pull/4266#discussion_r360015827

* replace else with else if

cmd/tendermint/commands/debug/kill.go

@@ -124,8 +124,7 @@ func killProc(pid uint64, dir string) error {
 		p, err := os.FindProcess(os.Getpid())
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "failed to find PID to kill Tendermint process: %s", err)
-		}
-		if err = p.Signal(syscall.SIGABRT); err != nil {
+		} else if err = p.Signal(syscall.SIGABRT); err != nil {
 			fmt.Fprintf(os.Stderr, "failed to kill Tendermint process: %s", err)
 		}