Add test for crashing

Tests/StencilTests/LexerSpec.swift

@@ -61,12 +61,17 @@ class LexerTests: XCTestCase {
 
       $0.it("can tokenize an unclosed block") {
         let lexer = Lexer(templateString: "{%}")
-        let _ = lexer.tokenize()
+        _ = lexer.tokenize()
+      }
+
+      $0.it("can tokenize incorrect syntax without crashing") {
+        let lexer = Lexer(templateString: "func some() {{% if %}")
+        _ = lexer.tokenize()
       }
 
       $0.it("can tokenize an empty variable") {
         let lexer = Lexer(templateString: "{{}}")
-        let _ = lexer.tokenize()
+        _ = lexer.tokenize()
       }
 
       $0.it("can tokenize with new lines") {
@@ -95,3 +100,4 @@ class LexerTests: XCTestCase {
       }
     }
   }
+}