Fix rare undefined offset errors in ConcatenationSpacingSniff

This can happen when PHPCS runs on a file that is currently being worked on, but not yet completed. The file might end with a dot. We can not assume there are always 2 more tokens after a dot.

Author: thiemowmde

src/Standards/Squiz/Sniffs/Strings/ConcatenationSpacingSniff.php

@@ -55,6 +55,10 @@ public function register()
     public function process(File $phpcsFile, $stackPtr)
     {
         $tokens = $phpcsFile->getTokens();
+        if (isset($tokens[($stackPtr + 2)]) === false) {
+            // Syntax error or live coding, bow out.
+            return;
+        }
 
         $ignoreBefore = false;
         $prev         = $phpcsFile->findPrevious(Tokens::$emptyTokens, ($stackPtr - 1), null, true);