syntax: fix hang with newlines in param exps

Newlines are not skipped in some scenarios inside ${}, but we were
refusing to tokenize them into literals. This meant that we were forever
trying to get past those bytes.

syntax/lexer.go

@@ -622,7 +622,9 @@ loop:
 			}
 			continue
 		case '\n':
-			if q != sglQuotes {
+			switch q {
+			case sglQuotes, paramExpRepl, paramExpExp:
+			default:
 				break loop
 			}
 			p.f.Lines = append(p.f.Lines, p.npos+1)

syntax/parser_test.go

@@ -912,6 +912,14 @@ var bashTests = []errorCase{
 		"echo ${foo[]}",
 		`1:11: [ must be followed by an expression`,
 	},
+	{
+		"echo ${a/\n",
+		`1:6: reached EOF without matching ${ with }`,
+	},
+	{
+		"echo ${a-\n",
+		`1:6: reached EOF without matching ${ with }`,
+	},
 	{
 		"echo ${foo:",
 		`1:11: : must be followed by an expression`,