proc: when stepping set condition on thread ID if there is no curg (#3475)

If there is no current goroutine when 'next', 'step' or 'stepout' are
used set a condition that the thread ID should stay the same instead.
This makes stepping work for multithreaded C programs or Go programs
that have threads started by cgo code.

Fixes #3262

pkg/astutil/astutil.go

@@ -30,11 +30,17 @@ func Int(n int64) *ast.BasicLit {
 }
 
 // And returns an expression evaluating 'x && y'.
-func And(x, y ast.Expr) *ast.BinaryExpr {
+func And(x, y ast.Expr) ast.Expr {
+	if x == nil || y == nil {
+		return nil
+	}
 	return &ast.BinaryExpr{Op: token.LAND, X: x, Y: y}
 }
 
 // Or returns an expression evaluating 'x || y'.
-func Or(x, y ast.Expr) *ast.BinaryExpr {
+func Or(x, y ast.Expr) ast.Expr {
+	if x == nil || y == nil {
+		return nil
+	}
 	return &ast.BinaryExpr{Op: token.LOR, X: x, Y: y}
 }