[shotwell] Fix JPEG file detection

[Jens Georg <jensgeorg src gnome org>]

commit f468f4a53b11e02c7d101ccae1536207b2cf7e51
Author: Jens Georg <mail jensge org>
Date:   Wed Oct 23 21:51:22 2019 +0200

    Fix JPEG file detection

 src/photos/JfifSupport.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/photos/JfifSupport.vala b/src/photos/JfifSupport.vala
index 36450bd0..2f33849f 100644
--- a/src/photos/JfifSupport.vala
+++ b/src/photos/JfifSupport.vala
@@ -285,7 +285,7 @@ namespace Jpeg {
         marker = (Jpeg.Marker) dins.read_byte();
         if ((marker == Jpeg.Marker.SOI) || (marker == Jpeg.Marker.EOI)) {
             // no length
-            return -1;
+            return 0;
         }
         
         uint16 length = dins.read_uint16();