internal/lsp/cache: fix crash fixing curlies near EOF

We were crashing in cases like:

1:    func foo() {
2:     if b<> <EOF>

We were trying to get the line start position for line 3, but there is
no line 3. Fix by bailing out early if we are the last line in the
file because there is nothing to fix in that case.

Fixes golang/go#37226.

Change-Id: I4ad5746d7b55bdcc2de57c04e972c15a61084faa
Reviewed-on: https://go-review.googlesource.com/c/tools/+/219498
Run-TryBot: Muir Manders <muir@mnd.rs>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/cache/parse.go

@@ -328,12 +328,19 @@ func fixMissingCurlies(f *ast.File, b *ast.BlockStmt, parent ast.Node, tok *toke
 		}
 	}
 
+	parentLine := tok.Line(parent.Pos())
+
+	if parentLine >= tok.LineCount() {
+		// If we are the last line in the file, no need to fix anything.
+		return nil
+	}
+
 	// Insert curlies at the end of parent's starting line. The parent
 	// is the statement that contains the block, e.g. *ast.IfStmt. The
 	// block's Pos()/End() can't be relied upon because they are based
 	// on the (missing) curly braces. We assume the statement is a
 	// single line for now and try sticking the curly braces at the end.
-	insertPos := tok.LineStart(tok.Line(parent.Pos())+1) - 1
+	insertPos := tok.LineStart(parentLine+1) - 1
 
 	// Scootch position backwards until it's not in a comment. For example:
 	//

internal/lsp/testdata/lsp/primarymod/danglingstmt/dangling_if_eof.go

@@ -0,0 +1,8 @@
+package danglingstmt
+
+func bar5() bool { //@item(danglingBar5, "bar5", "func() bool", "func")
+	return true
+}
+
+func _() {
+	if b //@rank(" //", danglingBar5)

internal/lsp/testdata/lsp/summary.txt.golden

@@ -5,7 +5,7 @@ CompletionSnippetCount = 67
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 98
+RankedCompletionsCount = 99
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 38
 FoldingRangesCount = 2