cmd/guru: fix nil dereference in describe

+ test

Fixes issue golang/go#14579

Change-Id: I6fd56f10c170027f06917935ccbc28490d413d8a
Reviewed-on: https://go-review.googlesource.com/20100
Reviewed-by: Michael Matloob <matloob@golang.org>

cmd/guru/describe.go

@@ -324,6 +324,9 @@ func describeValue(qpos *queryPos, path []ast.Node) (*describeValueResult, error
 	}
 
 	t := qpos.info.TypeOf(expr)
+	if t == nil {
+		t = types.Typ[types.Invalid]
+	}
 	constVal := qpos.info.Types[expr].Value
 
 	return &describeValueResult{

cmd/guru/testdata/src/describe/main.go

@@ -86,6 +86,8 @@ func main() { // @describe func-def-main "main"
 	var _ int // @describe var-def-blank "_"
 
 	var _ lib.Outer // @describe lib-outer "Outer"
+
+	unknown() // @describe call-unknown "\\("
 }
 
 type I interface { // @describe def-iface-I "I"

cmd/guru/testdata/src/describe/main.golden

@@ -197,6 +197,9 @@ Fields:
 	inner.C           bool
 	inner.recursive.E bool
 
+-------- @describe call-unknown --------
+function call (or conversion) of type invalid type
+
 -------- @describe def-iface-I --------
 definition of type I (size 16, align 8)
 Method set: