gopls/internal/golang/completion: ensure expectedCompositeLiteralType arg is not nil

It was possible for expectedCompositeLiteralType to be given a nil
*compLitInfo. Check for nilness outside the offending call site and skip
the call.

Fixes golang/go#72136.

Change-Id: Idc661145ae409a19909e0b4e1f74163acb11f5b5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/659375
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: jba

gopls/internal/golang/completion/completion.go

@@ -2001,6 +2001,8 @@ func (c *completer) structLiteralFieldName(ctx context.Context) error {
 
 // enclosingCompositeLiteral returns information about the composite literal enclosing the
 // position.
+// It returns nil on failure; for example, if there is no type information for a
+// node on path.
 func enclosingCompositeLiteral(path []ast.Node, pos token.Pos, info *types.Info) *compLitInfo {
 	for _, n := range path {
 		switch n := n.(type) {
@@ -2565,7 +2567,7 @@ func inferExpectedResultTypes(c *completer, callNodeIdx int) []types.Type {
 	switch node := c.path[callNodeIdx+1].(type) {
 	case *ast.KeyValueExpr:
 		enclosingCompositeLiteral := enclosingCompositeLiteral(c.path[callNodeIdx:], callNode.Pos(), c.pkg.TypesInfo())
-		if !wantStructFieldCompletions(enclosingCompositeLiteral) {
+		if enclosingCompositeLiteral != nil && !wantStructFieldCompletions(enclosingCompositeLiteral) {
 			expectedResults = append(expectedResults, expectedCompositeLiteralType(enclosingCompositeLiteral, callNode.Pos()))
 		}
 	case *ast.AssignStmt: