run1 can be <0 bugfix (found by fabrice)

Originally committed as revision 661 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h263.c

@@ -2966,7 +2966,7 @@ static inline int mpeg4_decode_block(MpegEncContext * s, DCTELEM * block,
                                 fprintf(stderr, "illegal 3. esc, esc 1 encoding possible\n");
                                 return DECODING_AC_LOST;
                             }
-                            if(abs_level <= rl->max_level[last][run1]){
+                            if(run1 >= 0 && abs_level <= rl->max_level[last][run1]){
                                 fprintf(stderr, "illegal 3. esc, esc 2 encoding possible\n");
                                 return DECODING_AC_LOST;
                             }