parser: don't hang if heredoc errors

We would tokenize whatever bytes were left, which meant that we could be
overwriting tok = EOF with garbage. In this case, with RPAREN for ).

In some weird cases like this one, the parser would get confused and
loop forever.

parser/parser_test.go

@@ -554,6 +554,10 @@ var shellTests = []struct {
 		"<<EOF\n$(()a",
 		`2:1: reached EOF without matching $(( with ))`,
 	},
+	{
+		"<<EOF\n`))",
+		`2:2: ) can only be used to close a subshell`,
+	},
 	{
 		"echo ${foo",
 		`1:6: reached EOF without matching ${ with }`,

parser/tokenizer.go

@@ -126,6 +126,9 @@ skipSpace:
 			p.newLine = true
 			if len(p.heredocs) > p.buriedHdocs {
 				p.doHeredocs()
+				if p.tok == token.EOF {
+					return
+				}
 			}
 		case '\\':
 			if p.npos < len(p.src)-1 && p.src[p.npos+1] == '\n' {