Report/Code: fix fatal potential fatal error when combined with Diff report

Okay, so this is an awkward one.

If both the `Code` + the `Diff` report were requested + caching was turned on + the _order_ of the requested reports was `Code,Diff` (i.e. first the code report), the following fatal error which would occur:
```
Fatal error: Uncaught Error: Call to a member function getFixableCount() on null in path/to/PHP_CodeSniffer/src/Fixer.php:144
Stack trace:
#0 path/to/PHP_CodeSniffer/src/Reports/Diff.php(73): PHP_CodeSniffer\Fixer->fixFile()
#1 path/to/PHP_CodeSniffer/src/Reporter.php(285): PHP_CodeSniffer\Reports\Diff->generateFileReport(Array, Object(PHP_CodeSniffer\Files\LocalFile), true, 150)
#2 path/to/PHP_CodeSniffer/src/Runner.php(659): PHP_CodeSniffer\Reporter->cacheFileReport(Object(PHP_CodeSniffer\Files\LocalFile))
#3 path/to/PHP_CodeSniffer/src/Runner.php(400): PHP_CodeSniffer\Runner->processFile(Object(PHP_CodeSniffer\Files\LocalFile))
#4 path/to/PHP_CodeSniffer/src/Runner.php(119): PHP_CodeSniffer\Runner->run()
#5 path/to/PHP_CodeSniffer/bin/phpcs(30): PHP_CodeSniffer\Runner->runPHPCS()
#6 {main}
  thrown in path/to/PHP_CodeSniffer/src/Fixer.php on line 144
```

To reproduce the issue (on `master`):
1. Create a small test file like:
    ```php
    <?php

    echo 'hello'.callMe( $p );
    ```
2. Run the following command to initialize the cache:
    ```bash
    phpcs -ps ./test.php --standard=PSR2 --report=Code,Diff --cache
    ```
    All should be fine.
3. Now run the same command again and see the fatal error.

Some investigating later, it turns out that both the `Code` report, as well as the `Diff` report, re-parse the current file, with the `Diff` report initializing the fixer once the file has reparsed, but the `Code` report not doing so (as it doesn't need the fixer).
The problem with that is that the `Code` report basically leaves the `Fixer` in an invalid state, leading to the above error.

While it is up for debate whether reports should ever (be allowed to) re-parse files, for now, let's fix the fatal.

Re-evaluating the report setup should definitely be done, but should probably wait until the Reports classes have test coverage.

Author: jrfnl

src/Reports/Code.php

@@ -55,6 +55,10 @@ public function generateFileReport($report, File $phpcsFile, $showSources=false,
 
             try {
                 $phpcsFile->parse();
+
+                // Make sure the fixer is aware of the reparsed file to prevent a race-condition
+                // with the Diff report also re-parsing the file.
+                $phpcsFile->fixer->startFile($phpcsFile);
             } catch (Exception $e) {
                 // This is a second parse, so ignore exceptions.
                 // They would have been added to the file's error list already.