Explicitly check the parser for errors on peek

It's curious choice from the underlying API to generally return a
positive result on success, but on this case return true in an error
scenario.

Fixes #666

decode.go

@@ -100,7 +100,10 @@ func (p *parser) peek() yaml_event_type_t {
 	if p.event.typ != yaml_NO_EVENT {
 		return p.event.typ
 	}
-	if !yaml_parser_parse(&p.parser, &p.event) {
+	// It's curious choice from the underlying API to generally return a
+	// positive result on success, but on this case return true in an error
+	// scenario. This was the source of bugs in the past (issue #666).
+	if !yaml_parser_parse(&p.parser, &p.event) || p.parser.error != yaml_NO_ERROR {
 		p.fail()
 	}
 	return p.event.typ

decode_test.go

@@ -947,6 +947,7 @@ var unmarshalErrorTests = []struct {
 	{"%TAG !%79! tag:yaml.org,2002:\n---\nv: !%79!int '1'", "yaml: did not find expected whitespace"},
 	{"a:\n  1:\nb\n  2:", ".*could not find expected ':'"},
 	{"a: 1\nb: 2\nc 2\nd: 3\n", "^yaml: line 3: could not find expected ':'$"},
+	{"0: [:!00 \xef", "yaml: incomplete UTF-8 octet sequence"}, // Issue #666
 	{
 		"a: &a [00,00,00,00,00,00,00,00,00]\n" +
 			"b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]\n" +