Couple of edge cases in SourceResource. (#288)

* Potential read issue.

Reading a single `"0"` could potentially be interpreted as a false response per PHP's truth table, resulting in returning a `null`.

* `fseek()` can fail.

Should `fseek()` fail, we probably want to flag the error.

https://www.libvips.org/API/8.17/vfunc.Source.seek.html#return-value seems to suggest that a `-1` would be appropriate to return in such a case?

Author: adam-vessey

src/SourceResource.php

@@ -21,13 +21,16 @@ public function __construct($resource)
         parent::__construct();
 
         $this->onRead(static function (int $length) use (&$resource): ?string {
-            return fread($resource, $length) ?: null;
+            return (($read = fread($resource, $length)) !== '') ?
+                $read :
+                null;
         });
 
         if (stream_get_meta_data($resource)['seekable']) {
             $this->onSeek(static function (int $offset, int $whence) use (&$resource): int {
-                fseek($resource, $offset, $whence);
-                return ftell($resource);
+                return fseek($resource, $offset, $whence) === 0 ?
+                    ftell($resource) :
+                    -1;
             });
         }
     }