Race in agent logger when connection fails

Agent logger panics at agent_log.go:108 when multiple goroutines try to close same transport when upload errors are encountered.

goroutine 4713 [running]:
main.(*rpcLogWriter).sendWithRetry(0x14000290060, {0x102e57898, 0x14000100230}, {

{0x14000522c00, 0xb2b, 0xc00}, {0x0, 0x0}})
/var/lib/coin/tqtc-coin-ci/src/agent/agent_log.go:108+0x1ac
reflect.Value.call({0x102dbfd20?, 0x140004da1c0?, 0x13?}, {0x102cae62e, 0x4}, {0x1400040a090, 0x2, 0x2?})
/var/lib/coin/go120/src/reflect/value.go:586+0x87c
reflect.Value.Call({0x102dbfd20?, 0x140004da1c0?, 0x0?}, {0x1400040a090?, 0x14000676f18?, 0x102b42748?})
/var/lib/coin/go120/src/reflect/value.go:370+0x90
ciapp.WrapFuncGoroutine.func1()
/var/lib/coin/tqtc-coin-ci/src/ciapp/context.go:76+0x5bc
created by ciapp.WrapFuncGoroutine
/var/lib/coin/tqtc-coin-ci/src/ciapp/context.go:46+0xe4
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x18 pc=0x102ca1a3c]

goroutine 4721 [running]:
main.(*rpcLogWriter).sendWithRetry(0x14000290060, {0x102e57898, 0x140002122d0}, {{0x14000522c00, 0xb2b, 0xc00}

, {0x0, 0x0}})
/var/lib/coin/tqtc-coin-ci/src/agent/agent_log.go:108+0x1ac
reflect.Value.call({0x102dbfd20?, 0x1400017c1f0?, 0x13?}, {0x102cae62e, 0x4}, {0x140004f8030, 0x2, 0x2?})
/var/lib/coin/go120/src/reflect/value.go:586+0x87c
reflect.Value.Call({0x102dbfd20?, 0x1400017c1f0?, 0x0?}, {0x140004f8030?, 0x14000ae7f18?, 0x102b42748?})
/var/lib/coin/go120/src/reflect/value.go:370+0x90
ciapp.WrapFuncGoroutine.func1()
/var/lib/coin/tqtc-coin-ci/src/ciapp/context.go:76+0x5bc
created by ciapp.WrapFuncGoroutine
/var/lib/coin/tqtc-coin-ci/src/ciapp/context.go:46+0xe4