go/pointer: fix two crashes caused by 'print()'.

+ Test.

LGTM=crawshaw
R=crawshaw
CC=golang-codereviews
https://golang.org/cl/173540043

go/pointer/gen.go

diff --git a/go/pointer/gen.go b/go/pointer/gen.go
index 12f4d79..48ca368 100644
--- a/go/pointer/gen.go
+++ b/go/pointer/gen.go
@@ -525,7 +525,9 @@
 	case "print":
 		// In the tests, the probe might be the sole reference
 		// to its arg, so make sure we create nodes for it.
-		a.valueNode(call.Args[0])
+		if len(call.Args) > 0 {
+			a.valueNode(call.Args[0])
+		}
 
 	case "ssa:wrapnilchk":
 		a.copy(a.valueNode(instr.Value()), a.valueNode(call.Args[0]), 1)

go/pointer/pointer_test.go

diff --git a/go/pointer/pointer_test.go b/go/pointer/pointer_test.go
index aeffb5c..ed3ebb0 100644
--- a/go/pointer/pointer_test.go
+++ b/go/pointer/pointer_test.go
@@ -191,7 +191,8 @@
 			for _, b := range fn.Blocks {
 				for _, instr := range b.Instrs {
 					if instr, ok := instr.(ssa.CallInstruction); ok {
-						if b, ok := instr.Common().Value.(*ssa.Builtin); ok && b.Name() == "print" {
+						call := instr.Common()
+						if b, ok := call.Value.(*ssa.Builtin); ok && b.Name() == "print" && len(call.Args) == 1 {
 							probes[instr.Common()] = true
 						}
 					}

go/pointer/testdata/another.go

diff --git a/go/pointer/testdata/another.go b/go/pointer/testdata/another.go
index 443c94d..12ed690 100644
--- a/go/pointer/testdata/another.go
+++ b/go/pointer/testdata/another.go
@@ -31,4 +31,6 @@
 	// labels, even though it may contain pointers that do.
 	print(i)                 // @pointsto makeinterface:func(x int) int | makeinterface:func(x int, y int) | makeinterface:func(int, int) | makeinterface:int | makeinterface:main.S
 	print(i.(func(int) int)) // @pointsto main.incr
+
+	print() // regression test for crash
 }