internal/lsp/source/completion: avoid invalid AST in enclosingSignature

Guard against the panic in golang/go#49397, though it should not be
possible with well-formed AST.

Updates golang/go#49397

Change-Id: I3428504a4cfd361c1f51316d8c9aeee4aa9d7f42
Reviewed-on: https://go-review.googlesource.com/c/tools/+/364675
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

internal/lsp/source/completion/completion.go

@@ -1694,6 +1694,12 @@ func enclosingFunction(path []ast.Node, info *types.Info) *funcInfo {
 			}
 		case *ast.FuncLit:
 			if typ, ok := info.Types[t]; ok {
+				if sig, _ := typ.Type.(*types.Signature); sig == nil {
+					// golang/go#49397: it should not be possible, but we somehow arrived
+					// here with a non-signature type, most likely due to AST mangling
+					// such that node.Type is not a FuncType.
+					return nil
+				}
 				return &funcInfo{
 					sig:  typ.Type.(*types.Signature),
 					body: t.Body,