fix(lexer): Prevent deadlock with unclosed tag

Sources/Lexer.swift

@@ -90,6 +90,7 @@ class Scanner {
       index = content.index(after: index)
     }
 
+    content = ""
     return ""
   }
 

Tests/StencilTests/LexerSpec.swift

@@ -16,7 +16,7 @@ func testLexer() {
       let lexer = Lexer(templateString: "{# Comment #}")
       let tokens = lexer.tokenize()
 
-      try expect(tokens.count) == (1)
+      try expect(tokens.count) == 1
       try expect(tokens.first) == .comment(value: "Comment")
     }
 
@@ -28,6 +28,14 @@ func testLexer() {
       try expect(tokens.first) == .variable(value: "Variable")
     }
 
+    $0.it("can tokenize unclosed tag by ignoring it") {
+      let lexer = Lexer(templateString: "{{ thing")
+      let tokens = lexer.tokenize()
+
+      try expect(tokens.count) == 1
+      try expect(tokens.first) == .text(value: "")
+    }
+
     $0.it("can tokenize a mixture of content") {
       let lexer = Lexer(templateString: "My name is {{ name }}.")
       let tokens = lexer.tokenize()