fix(array test): Fail with missing separator

The toml-test generated test for an array with missing separators was
failing since the parser wasn't noticing missing separators.

This change leverages the zero value of `ast.Reference` (an int). The
var is now declared outside the while block, and re-zeroed if the
token is not a non-trailing comma.

The parser checks if the previous value of `valueRef` is zero. The
only time this check would pass is when the previous byte in the array
was a whitespace, which would only be the case when an array is
missing a separator.

Resolves: part of pelletier#613

parser.go

@@ -414,6 +414,7 @@ func (p *parser) parseValArray(b []byte) (ast.Reference, []byte, error) {
 
 	var lastChild ast.Reference
 
+	var valueRef ast.Reference
 	var err error
 	for len(b) > 0 {
 		b, err = p.parseOptionalWhitespaceCommentNewline(b)
@@ -439,14 +440,18 @@ func (p *parser) parseValArray(b []byte) (ast.Reference, []byte, error) {
 			if err != nil {
 				return parent, nil, err
 			}
+		} else {
+			valueRef = 0
 		}
 
 		// TOML allows trailing commas in arrays.
 		if len(b) > 0 && b[0] == ']' {
 			break
 		}
 
-		var valueRef ast.Reference
+		if !first && valueRef == 0 {
+			return parent, nil, newDecodeError(b[0:1], "array elements must be separated by commas")
+		}
 
 		valueRef, b, err = p.parseVal(b)
 		if err != nil {