bug: crash on new shell after a long, long session

[daonb]

from webexec.err:

[Fx] RUNNING
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x1054c8b44]

goroutine 8729368 [running]:
github.com/tuzig/webexec/peers.(*Pane).OnMessage(0x14002005600, {0x68?, {0x140036b205c?, 0x105014084?, 0x1056b8801?}})
	/Users/daonb/src/webexec/peers/pane.go:278 +0x34
main.handleAddPane.func1.1({0xf8?, {0x140036b205c?, 0x0?, 0x140045b6e70?}})
	/Users/daonb/src/webexec/handlers.go:190 +0xb8
github.com/pion/webrtc/v3.(*DataChannel).onMessage(0x140028de000?, {0xa0?, {0x140036b205c?, 0xffff?, 0x140004a17a8?}})
	/Users/daonb/go/pkg/mod/github.com/pion/webrtc/v3@v3.2.32/datachannel.go:300 +0x114
github.com/pion/webrtc/v3.(*DataChannel).readLoop(0x14000add440)
	/Users/daonb/go/pkg/mod/github.com/pion/webrtc/v3@v3.2.32/datachannel.go:378 +0x5c
created by github.com/pion/webrtc/v3.(*DataChannel).handleOpen in goroutine 8495940
	/Users/daonb/go/pkg/mod/github.com/pion/webrtc/v3@v3.2.32/datachannel.go:330 +0x1c4