Catch potential NULL return value from preg_replace_callback

For reference: issue #356

Author: theseer

src/collector/backend/SourceFileException.php

@@ -3,6 +3,6 @@
 
 class SourceFileException extends \Exception {
     public const BadEncoding = 1;
-
     public const InvalidDataBytes = 2;
+    public const RegexError = 3;
 }

src/collector/project/SourceFile.php

@@ -43,7 +43,7 @@ public function getSource(): string {
 
         $source = \file_get_contents($this->getPathname());
 
-        if ($source == '') {
+        if ($source === '') {
             $this->src = '';
 
             return '';
@@ -71,6 +71,13 @@ function (array $matches) {
             $source
         );
 
+        if ($this->src === null) {
+            throw new SourceFileException(
+                sprintf('Error %d while executing regular expression callback', \preg_last_error()),
+                SourceFileException::RegexError
+            );
+        }
+
         return $this->src;
     }