Avoid fixer conflict on parse error

Because PHP evaluates `false - 1` to `(int) -1`, the array look-up was looking
for `$tokens[-1]` which does not exist. This code change avoids that case by
detecting the parse error / live coding situation early.

Includes test.

Author: fredden

src/Standards/PSR12/Sniffs/Functions/ReturnTypeDeclarationSniff.php

@@ -61,6 +61,12 @@ public function process(File $phpcsFile, int $stackPtr)
             $returnType = $phpcsFile->findPrevious(T_NULLABLE, ($returnType - 1));
         }
 
+        $colon = $phpcsFile->findPrevious(T_COLON, $returnType, $tokens[$stackPtr]['parenthesis_closer']);
+        if ($colon === false) {
+            // Parse error / live coding.
+            return;
+        }
+
         if ($tokens[($returnType - 1)]['code'] !== T_WHITESPACE
             || $tokens[($returnType - 1)]['content'] !== ' '
             || $tokens[($returnType - 2)]['code'] !== T_COLON
@@ -83,7 +89,6 @@ public function process(File $phpcsFile, int $stackPtr)
             }
         }
 
-        $colon = $phpcsFile->findPrevious(T_COLON, $returnType);
         if ($tokens[($colon - 1)]['code'] !== T_CLOSE_PARENTHESIS) {
             $error = 'There must not be a space before the colon in a return type declaration';
             $prev  = $phpcsFile->findPrevious(T_WHITESPACE, ($colon - 1), null, true);

src/Standards/PSR12/Tests/Functions/ReturnTypeDeclarationUnitTest.2.inc

@@ -0,0 +1,4 @@
+<?php
+
+// Intentional parse error. There should be a colon between the function parameters and its return type.
+$closure = function () null { return null; };

src/Standards/PSR12/Tests/Functions/ReturnTypeDeclarationUnitTest.3.inc

@@ -0,0 +1,4 @@
+<?php
+
+// Intentional parse error. There should be a colon between the function 'use' and its return type.
+$closure = function ($foo) use ($bar) string { return 'baz'; };

src/Standards/PSR12/Tests/Functions/ReturnTypeDeclarationUnitTest.4.inc

@@ -0,0 +1,4 @@
+<?php
+
+// Intentional parse error. There should be a colon between the function parameters and its return type.
+function foo($obj = new DateTime(datetime: "now")) null {}