bug: signal SIGSEGV in case of missed database in connection string

[dcherednik]

Bug Report

YDB GO SDK version:

trunk

Environment

linux

Current behavior:

segfault

Expected behavior:

not segfault

Steps to reproduce:

• build example from ydb-go-sdk/examples/describe

• run:
  ./describe -ydb "grpc://127.0.0.1:28048"
  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xa18200]

goroutine 1 [running]:
github.com/ydb-platform/ydb-go-sdk/v3/internal/conn.(*Pool).Release(0x0, {0xece890, 0xc00017e910})
/home/ee8jsmrbc5rlptpnol4q/repos/ydb-go-sdk/internal/conn/pool.go:151 +0xa0
github.com/ydb-platform/ydb-go-sdk/v3.Open({0xece890, 0xc00017e910}, {0x7ffe82fde62b, 0x16}, {0xc0002b1f28, 0x1, 0xc0002b1ee8?})
/home/ee8jsmrbc5rlptpnol4q/repos/ydb-go-sdk/driver.go:277 +0x656
main.main()
/home/ee8jsmrbc5rlptpnol4q/repos/ydb-go-sdk/examples/describe/main.go:79 +0x9b
Related code: