internal/lsp: handle nil pointer in fixInitStmt

I'm going to make a follow-up CL to create a source.Offset function
that will always check inRange, and we should use that everywhere
instead of token.Offset, which is pretty prone to panicking.

Fixes golang/go#48763

Change-Id: Ia81309400d15a28c133f4b3d41c6239231c2532d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/353889
Trust: Rebecca Stambler <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: stamblerre

internal/lsp/cache/parse.go

@@ -995,6 +995,10 @@ func fixInitStmt(bad *ast.BadExpr, parent ast.Node, tok *token.File, src []byte)
 	}
 
 	// Try to extract a statement from the BadExpr.
+	// Make sure that the positions are in range first.
+	if !source.InRange(tok, bad.Pos()) || !source.InRange(tok, bad.End()-1) {
+		return
+	}
 	stmtBytes := src[tok.Offset(bad.Pos()) : tok.Offset(bad.End()-1)+1]
 	stmt, err := parseStmt(bad.Pos(), stmtBytes)
 	if err != nil {