fixed a small oob bug with extracting code (#696)

* happens in rare conditions when the message is ````` or similar, i.e. more than three backticks

Author: Zabuzard

application/src/main/java/org/togetherjava/tjbot/commands/utils/MessageUtils.java

@@ -180,15 +180,15 @@ public static Optional<CodeFence> extractCode(String fullMessage) {
             return Optional.empty();
         }
 
-        int codeFenceEnd = fullMessage.indexOf(CODE_FENCE_SYMBOL, codeFenceStart + 1);
+        int languageStart = codeFenceStart + CODE_FENCE_SYMBOL.length();
+        int codeFenceEnd = fullMessage.indexOf(CODE_FENCE_SYMBOL, languageStart);
         if (codeFenceEnd == -1) {
             return Optional.empty();
         }
 
         // Language is between ``` and newline, no spaces allowed, like ```java
         // Look for the next newline and then assert no space between
         String language = null;
-        int languageStart = codeFenceStart + CODE_FENCE_SYMBOL.length();
         int languageEnd = fullMessage.indexOf('\n', codeFenceStart);
         if (languageEnd != -1) {
             String languageCandidate = fullMessage.substring(languageStart, languageEnd);