[tracker] Fix crash when EXIF focal length value is empty

[Ivan Frade <ifrade src gnome org>]

commit b3ef621a4d70e25893c69d30bbe2a47d1bfed573
Author: Rene Stadler <mail renestadler de>
Date:   Mon Nov 30 14:58:59 2009 +0200

    Fix crash when EXIF focal length value is empty

 src/tracker-extract/tracker-exif.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/src/tracker-extract/tracker-exif.c b/src/tracker-extract/tracker-exif.c
index e67aefa..6d84d62 100644
--- a/src/tracker-extract/tracker-exif.c
+++ b/src/tracker-extract/tracker-exif.c
@@ -92,8 +92,14 @@ get_focal_length(ExifData *exif, ExifTag tag)
 
 	if (entry) {
 		gchar buf[1024];
+		const gchar *end;
 		exif_entry_get_value (entry, buf, 1024);
-		return g_strndup (buf, strstr (buf, " mm") - buf);
+		end = g_strstr_len (buf, 1024, " mm");
+		if (end) {
+			return g_strndup (buf, end - buf);
+		} else {
+			return NULL;
+		}
 	}
 
 	return NULL;