cmd/compile/internal/syntax: don't panic if no error handler is provided

If no error handler is provided, terminate parsing with first error
and report that error.

Fixes #17697.

Change-Id: I9070faf7239bd53725de141507912b92ded3474b
Reviewed-on: https://go-review.googlesource.com/32456
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/parser_test.go

@@ -155,3 +155,10 @@ func verifyPrint(filename string, ast1 *File) {
 		panic("not equal")
 	}
 }
+
+func TestIssue17697(t *testing.T) {
+	_, err := ReadBytes(nil, nil, nil, 0) // return with parser error, don't panic
+	if err == nil {
+		t.Errorf("no error reported")
+	}
+}