Fix a crash when decoding tiff by moving check

for invalid header up before reading data.
Fixes issue 2500.

Patch by Daniel Kang, daniel.d.kang at gmail

Originally committed as revision 26248 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/tiff.c

@@ -263,6 +263,10 @@ static int tiff_decode_tag(TiffContext *s, const uint8_t *start, const uint8_t *
         s->height = value;
         break;
     case TIFF_BPP:
+        if(count > 4){
+            av_log(s->avctx, AV_LOG_ERROR, "This format is not supported (bpp=%d, %d components)\n", s->bpp, count);
+            return -1;
+        }
         if(count == 1) s->bpp = value;
         else{
             switch(type){
@@ -278,10 +282,6 @@ static int tiff_decode_tag(TiffContext *s, const uint8_t *start, const uint8_t *
                 s->bpp = -1;
             }
         }
-        if(count > 4){
-            av_log(s->avctx, AV_LOG_ERROR, "This format is not supported (bpp=%d, %d components)\n", s->bpp, count);
-            return -1;
-        }
         switch(s->bpp*10 + count){
         case 11:
             s->avctx->pix_fmt = PIX_FMT_MONOBLACK;