targa: Do not read colormap from files without one.

This is necessary because some programs writing TGA files forget
to initialize parts of the header they don't care about, resulting
in "random" data there.
The new behaviour is consistent with other programs, e.g. tgatoppm.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/targa.c

@@ -109,17 +109,22 @@ static int decode_frame(AVCodecContext *avctx,
     AVFrame * const p= (AVFrame*)&s->picture;
     uint8_t *dst;
     int stride;
-    int idlen, compr, y, w, h, bpp, flags;
+    int idlen, pal, compr, y, w, h, bpp, flags;
     int first_clr, colors, csize;
 
     /* parse image header */
     CHECK_BUFFER_SIZE(buf, buf_end, 18, "header");
     idlen = *buf++;
-    buf++; /* pal */
+    pal = *buf++;
     compr = *buf++;
     first_clr = bytestream_get_le16(&buf);
     colors = bytestream_get_le16(&buf);
     csize = *buf++;
+    if (!pal && (first_clr || colors || csize)) {
+        av_log(avctx, AV_LOG_WARNING, "File without colormap has colormap information set.\n");
+        // specification says we should ignore those value in this case
+        first_clr = colors = csize = 0;
+    }
     buf += 2; /* x */
     y = bytestream_get_le16(&buf);
     w = bytestream_get_le16(&buf);