internal/lsp: fix panic in computing file version
internal/lsp: fix panic in computing file version

Fixes golang/go#32791

Change-Id: I04fc3de5ed94137424a32f3e7200ba72d3913329
Reviewed-on: https://go-review.googlesource.com/c/tools/+/183984
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools