rpc: fix getapplicationlog RPC handler
Fixes the following panic:
```
2020/12/22 18:16:09 http: panic serving 127.0.0.1:50228: runtime error: invalid memory address or nil pointer dereference
goroutine 4043 [running]:
net/http.(*conn).serve.func1(0xc00094c960)
net/http/server.go:1772 +0x139
panic(0xcd9b40, 0x16a94e0)
runtime/panic.go:973 +0x396
github.com/nspcc-dev/neo-go/pkg/rpc/server.(*Server).getApplicationLog(0xc000094ea0, 0xc000472d20, 0x2, 0x4, 0xc0000be228, 0xc0007ad601, 0x28)
github.com/nspcc-dev/neo-go/pkg/rpc/server/server.go:542 +0xac
github.com/nspcc-dev/neo-go/pkg/rpc/server.(*Server).handleIn(0xc000094ea0, 0xc000089770, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/rpc/server/server.go:326 +0x981
github.com/nspcc-dev/neo-go/pkg/rpc/server.(*Server).handleRequest(0xc000094ea0, 0xc001bccba0, 0x0, 0x0, 0x0)
github.com/nspcc-dev/neo-go/pkg/rpc/server/server.go:296 +0x26a
github.com/nspcc-dev/neo-go/pkg/rpc/server.(*Server).handleHTTPRequest(0xc000094ea0, 0x1071f40, 0xc000b089a0, 0xc00053c200)
github.com/nspcc-dev/neo-go/pkg/rpc/server/server.go:290 +0x91b
net/http.HandlerFunc.ServeHTTP(0xc0004e61b0, 0x1071f40, 0xc000b089a0, 0xc00053c200)
net/http/server.go:2012 +0x44
net/http.serverHandler.ServeHTTP(0xc0000d2ee0, 0x1071f40, 0xc000b089a0, 0xc00053c200)
net/http/server.go:2807 +0xa3
net/http.(*conn).serve(0xc00094c960, 0x10749c0, 0xc0006ae980)
net/http/server.go:1895 +0x86c
created by net/http.(*Server).Serve
net/http/server.go:2933 +0x35c
```
be0609cc7a