Reject text files resembling TGA images

Fixes issue #295

The imlib2 TGA loader returns an imlib image handle without any actual data
when given a text file like this:

    T
    Content-Type: application/javascript
    Content-Length: 3836
    Last-Modified: Wed, 23 Sep 2015 12:25:47 GMT
    Etag: "56029a4b-efc"
    Expires: Sat, 20 Aug 2016 15:14:33 GMT
    Cache-Control: max-age=604800, public
    Accept-Ranges: bytes

Fortunately, `imlib_image_get_data()` returns NULL in this case, so that we can
use it as an additional check when opening files.

1 files changed, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 6113957..6c26d3c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-VERSION = 24
+VERSION = git-20171123
 
 srcdir = .
 VPATH = $(srcdir)