[gopls-release-branch.0.4] internal/lsp/source: add extra nil check in searchForEnclosing

I still keep seeing this crash too, even after CL 244841.

Fixes golang/go#40464

Change-Id: Ic587045e65f34c24bd6df452e24517fd90e36bbe
Reviewed-on: https://go-review.googlesource.com/c/tools/+/245440
Run-TryBot: Rebecca Stambler <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
(cherry picked from commit df70183)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/245457

Author: stamblerre

internal/lsp/source/identifier.go

@@ -256,6 +256,9 @@ func searchForEnclosing(info *types.Info, path []ast.Node) types.Type {
 						}
 					}
 				}
+				if exported == nil {
+					return nil
+				}
 				return exported
 			}
 		case *ast.CompositeLit: