gopls/internal/golang: fix nil panic in InlayHint

Fixes golang/go#67142

Change-Id: Iee99c1fb0378c409bbbe803ad66b99a068e6d4bc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/582957
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

gopls/internal/golang/inlay_hint.go

@@ -139,7 +139,11 @@ func parameterNames(node ast.Node, m *protocol.Mapper, tf *token.File, info *typ
 	if !ok {
 		return nil
 	}
-	signature, ok := typeparams.CoreType(info.TypeOf(callExpr.Fun)).(*types.Signature)
+	t := info.TypeOf(callExpr.Fun)
+	if t == nil {
+		return nil
+	}
+	signature, ok := typeparams.CoreType(t).(*types.Signature)
 	if !ok {
 		return nil
 	}

gopls/internal/test/marker/testdata/inlayhints/issue67142.txt

@@ -0,0 +1,36 @@
+Regression test for golang/go#67142.
+
+-- flags --
+-ignore_extra_diags
+-min_go=go1.21
+
+-- settings.json --
+{
+	"hints": {
+		"assignVariableTypes": true,
+		"compositeLiteralFields": true,
+		"compositeLiteralTypes": true,
+		"constantValues": true,
+		"functionTypeParameters": true,
+		"parameterNames": true,
+		"rangeVariabletypes": true
+	}
+}
+
+-- go.mod --
+module w
+
+go 1.21.9
+
+-- p.go --
+//@inlayhints(out)
+package p
+
+var _ = rand.Float64() 
+
+-- @out --
+//@inlayhints(out)
+package p
+
+var _ = rand.Float64() 
+