WAT: Fix lexing of block comments

Close #164

Author: kateinoigakukun

Sources/WAT/Lexer.swift

@@ -306,7 +306,10 @@ struct Lexer {
     private mutating func lexBlockComment() throws -> TokenKind {
         var level = 1
         while true {
-            switch try cursor.next() {
+            guard let char = try cursor.next() else {
+                throw cursor.unexpectedEof()
+            }
+            switch char {
             case "(":
                 if try cursor.peek() == ";" {
                     // Nested comment block

Tests/WATTests/LexerTests.swift

@@ -25,6 +25,7 @@ class LexerTests: XCTestCase {
     }
 
     func testLexComment() {
+        XCTAssertEqual(try collectToken("(; foo ;)"), [.blockComment])
         try XCTAssertEqual(
             collectToken(
                 """
@@ -40,6 +41,12 @@ class LexerTests: XCTestCase {
 
     }
 
+    func testLexBrokenComment() {
+        XCTAssertThrowsError(try collectToken("(;)"))
+        XCTAssertThrowsError(try collectToken("(; foo )"))
+        XCTAssertThrowsError(try collectToken(";)"))
+    }
+
     func testLexIdAndString() throws {
         try XCTAssertEqual(collectToken("$foo"), [.id])
         try XCTAssertEqual(collectToken("\"foo\""), [.string(Array("foo".utf8))])