Server Crash when asking for completion at the beginning of a line.

[slrtbtfs]

Found by @metalmatze

Stacktace:

[Error - 3:09:45 PM] Request textDocument/completion failed.
extension.js:57
  Message: method "textDocument/completion" did not reply
  Code: -32603 
extension.js:57
panic: illegal line number

goroutine 10 [running]:

extension.js:49
go/token.(*File).LineStart(0xc00011a0c0, 0x2, 0x0)
	/usr/lib/golang/src/go/token/position.go:222 +0xf6
github.com/slrtbtfs/promql-lsp/langserver/cache.(*Document).ProtocolPositionToTokenPos(0xc000239c70, 0xa61100, 0xc00021c580, 0x3ff0000000000000, 0x0, 0x0, 0x0, 0x0)
	/home/slrtbtfs/git/github.com/slrtbtfs/promql-lsp/langserver/cache/position.go:89 +0x105
github.com/slrtbtfs/promql-lsp/langserver.(*Server).Completion(0xc000170a80, 0xa61100, 0xc000038c00, 0xc00021c5c0, 0xc00021c5c0, 0x0, 0x0)
	/home/slrtbtfs/git/github.com/slrtbtfs/promql-lsp/langserver/completion.go:40 +0x11a
github.com/slrtbtfs/promql-lsp/vendored/go-tools/lsp/protocol.serverHandler.Deliver(0xa704c0, 0xc000170a80, 0xa61100, 0xc000038c00, 0xc000038c40, 0x0, 0x1)
	/home/slrtbtfs/git/github.com/slrtbtfs/promql-lsp/vendored/go-tools/lsp/protocol/tsserver.go:310 +0x222a
github.com/slrtbtfs/promql-lsp/vendored/go-tools/jsonrpc2.(*Conn).Run.func1(0xc00002e300, 0xc000038c40, 0xc0001c06c0, 0xa61100, 0xc000038c00, 0x0, 0x0, 0xc000071280)
	/home/slrtbtfs/git/github.com/slrtbtfs/promql-lsp/vendored/go-tools/jsonrpc2/jsonrpc2.go:370 +0x170
created by github.com/slrtbtfs/promql-lsp/vendored/go-tools/jsonrpc2.(*Conn).Run
	/home/slrtbtfs/git/github.com/slrtbtfs/promql-lsp/vendored/go-tools/jsonrpc2/jsonrpc2.go:354 +0x877

extension.js:49
[Error - 3:09:45 PM] Connection to server got closed. Server will not be restarted.

This is a special case of the problem described in #22.

[slrtbtfs]

The crash only happens if it is the last line in the file.