PHP Tokenizer: fix "undefined offset" notice during live coding / arrow functions

Defensive coding for the arrow functions backfill. This fixes a number of "undefined index" PHP notices when arrow functions are being declared during live coding.

```
Notice: Undefined offset: 561 in /path/to/PHP_CodeSniffer/src/Tokenizers/PHP.php on line 1807
Notice: Undefined offset: 561 in /path/to/PHP_CodeSniffer/src/Tokenizers/PHP.php on line 1808
Notice: Undefined offset: 561 in /path/to/PHP_CodeSniffer/src/Tokenizers/PHP.php on line 1815
```

It also fixes a bug where an inner for loop was using the wrong variable for the condition part.

Includes adjusted unit test + additional unit test covering all changes.

Includes minor documentation fix.

Author: jrfnl

src/Tokenizers/PHP.php

@@ -1801,9 +1801,9 @@ protected function processAdditional()
                 }//end if
 
                 continue;
-            } else if ($this->tokens[$i]['code'] === T_FN) {
+            } else if ($this->tokens[$i]['code'] === T_FN && isset($this->tokens[($i + 1)]) === true) {
                 // Possible arrow function.
-                for ($x = ($i + 1); $i < $numTokens; $x++) {
+                for ($x = ($i + 1); $x < $numTokens; $x++) {
                     if (isset(Util\Tokens::$emptyTokens[$this->tokens[$x]['code']]) === false
                         && $this->tokens[$x]['code'] !== T_BITWISE_AND
                     ) {
@@ -1812,7 +1812,7 @@ protected function processAdditional()
                     }
                 }
 
-                if ($this->tokens[$x]['code'] === T_OPEN_PARENTHESIS) {
+                if (isset($this->tokens[$x]) === true && $this->tokens[$x]['code'] === T_OPEN_PARENTHESIS) {
                     $ignore  = Util\Tokens::$emptyTokens;
                     $ignore += [
                         T_STRING       => T_STRING,

tests/Core/Tokenizer/BackfillFnTokenTest.inc

@@ -69,3 +69,7 @@ fn(callable $a) : callable => $a;
 
 /* testTernary */
 $fn = fn($a) => $a ? /* testTernaryThen */ fn() : string => 'a' : /* testTernaryElse */ fn() : string => 'b';
+
+/* testLiveCoding */
+// Intentional parse error. This has to be the last test in the file.
+$fn = fn

tests/Core/Tokenizer/BackfillFnTokenTest.php

@@ -552,7 +552,31 @@ public function testTernary()
 
 
     /**
-     * Test that anonymous class tokens without parenthesis do not get assigned a parenthesis owner.
+     * Test that the backfill presumes T_FN during live coding, but doesn't set the additional index keys.
+     *
+     * @covers PHP_CodeSniffer\Tokenizers\PHP::processAdditional
+     *
+     * @return void
+     */
+    public function testLiveCoding()
+    {
+        $tokens = self::$phpcsFile->getTokens();
+
+        $token = $this->getTargetToken('/* testLiveCoding */', [T_STRING, T_FN]);
+        $this->assertSame($tokens[$token]['code'], T_FN, 'Token not tokenized as T_FN');
+
+        $this->assertArrayNotHasKey('scope_condition', $tokens[$token], 'Scope condition is set');
+        $this->assertArrayNotHasKey('scope_opener', $tokens[$token], 'Scope opener is set');
+        $this->assertArrayNotHasKey('scope_closer', $tokens[$token], 'Scope closer is set');
+        $this->assertArrayNotHasKey('parenthesis_owner', $tokens[$token], 'Parenthesis owner is set');
+        $this->assertArrayNotHasKey('parenthesis_opener', $tokens[$token], 'Parenthesis opener is set');
+        $this->assertArrayNotHasKey('parenthesis_closer', $tokens[$token], 'Parenthesis closer is set');
+
+    }//end testLiveCoding()
+
+
+    /**
+     * Helper function to check that all token keys are correctly set for T_FN tokens.
      *
      * @param string $token The T_FN token to check.
      *