4l: TIFF stores short strings inside tag, do not interpret it is as an offset

Fixes issue 753

Originally committed as revision 16189 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/tiff.c

@@ -176,6 +176,11 @@ static int tiff_decode_tag(TiffContext *s, const uint8_t *start, const uint8_t *
             value = off;
             buf = NULL;
             break;
+        case TIFF_STRING:
+            if(count <= 4){
+                buf -= 4;
+                break;
+            }
         default:
             value = -1;
             buf = start + off;