go/types: handle nil pointer when panic is written outside of a function

The current implementation crashes when someone writes a panic outside of
a function, which makes sense since that is broken code. This fix allows
one to type-check broken code.

Updates #22467

Change-Id: I81b90dbd918162a20c60a821340898eaf02e648d
Reviewed-on: https://go-review.googlesource.com/132235Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/api_test.go

@@ -261,6 +261,8 @@ func TestTypesInfo(t *testing.T) {
 		{`package x0; func _() { var x struct {f string}; x.f := 0 }`, `x.f`, `string`},
 		{`package x1; func _() { var z string; type x struct {f string}; y := &x{q: z}}`, `z`, `string`},
 		{`package x2; func _() { var a, b string; type x struct {f string}; z := &x{f: a; f: b;}}`, `b`, `string`},
+		{`package x3; var x = panic("");`, `panic`, `func(interface{})`},
+		{`package x4; func _() { panic("") }`, `panic`, `func(interface{})`},
 	}
 
 	for _, test := range tests {

src/go/types/builtins.go

@@ -476,7 +476,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		// panic(x)
 		// record panic call if inside a function with result parameters
 		// (for use in Checker.isTerminating)
-		if check.sig.results.Len() > 0 {
+		if check.sig != nil && check.sig.results.Len() > 0 {
 			// function has result parameters
 			p := check.isPanic
 			if p == nil {