go/parser: make sure we have a valid AST when 'if' condition is missing

This prevents a crash in go/types due to a nil condition in an 'if'
statement. There's more we can do to make go/types more robust but
this will address the immediate cause and also makes sure that the
parser returns a valid AST in this case.

Fixes #25438.

Change-Id: Ie55dc2c722352a5ecb17af6a16983741e8a8b515
Reviewed-on: https://go-review.googlesource.com/113735
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/parser/parser.go

@@ -1830,6 +1830,7 @@ func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
 func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
 	if p.tok == token.LBRACE {
 		p.error(p.pos, "missing condition in if statement")
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
 		return
 	}
 	// p.tok != token.LBRACE
@@ -1877,6 +1878,11 @@ func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
 		}
 	}
 
+	// make sure we have a valid AST
+	if cond == nil {
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
+	}
+
 	p.exprLev = outer
 	return
 }

src/go/types/testdata/issues.src

@@ -240,3 +240,11 @@ func issue24140(x interface{}) int {
                 panic(0)
         }
 }
+
+// Test that we don't crash when the 'if' condition is missing.
+func issue25438() {
+	if { /* ERROR missing condition */ }
+	if x := 0; /* ERROR missing condition */ { _ = x }
+	if
+	{ /* ERROR missing condition */ }
+}