reduce len checks, simplifies code

there is a very small chance that a stream which is damaged && unseekable && very small packet && damage to the flags && unlucky buffer state could have a valid packet after a damaged packet lost, but i think this is so unlikely that the simplification outweights it
mans if you disagree with any commit to mpeg.c just say so and ill revert it

Originally committed as revision 7562 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mpeg.c

@@ -1498,20 +1498,14 @@ static int mpegps_read_pes_header(AVFormatContext *s,
     }
     if ((c & 0xc0) == 0x40) {
         /* buffer scale & size */
-        if (len < 2)
-            goto error_redo;
         get_byte(&s->pb);
         c = get_byte(&s->pb);
         len -= 2;
     }
     if ((c & 0xf0) == 0x20) {
-        if (len < 4)
-            goto error_redo;
         dts = pts = get_pts(&s->pb, c);
         len -= 4;
     } else if ((c & 0xf0) == 0x30) {
-        if (len < 9)
-            goto error_redo;
         pts = get_pts(&s->pb, c);
         dts = get_pts(&s->pb, -1);
         len -= 9;
@@ -1530,15 +1524,11 @@ static int mpegps_read_pes_header(AVFormatContext *s,
             goto error_redo;
         if ((flags & 0xc0) == 0x80) {
             dts = pts = get_pts(&s->pb, -1);
-            if (header_len < 5)
-                goto error_redo;
             header_len -= 5;
             len -= 5;
         } if ((flags & 0xc0) == 0xc0) {
             pts = get_pts(&s->pb, -1);
             dts = get_pts(&s->pb, -1);
-            if (header_len < 10)
-                goto error_redo;
             header_len -= 10;
             len -= 10;
         }
@@ -1552,20 +1542,18 @@ static int mpegps_read_pes_header(AVFormatContext *s,
         goto redo;
 
     if (startcode == PRIVATE_STREAM_1 && !m->psm_es_type[startcode & 0xff]) {
-        if (len < 1)
-            goto error_redo;
         startcode = get_byte(&s->pb);
         len--;
         if (startcode >= 0x80 && startcode <= 0xbf) {
             /* audio: skip header */
-            if (len < 3)
-                goto error_redo;
             get_byte(&s->pb);
             get_byte(&s->pb);
             get_byte(&s->pb);
             len -= 3;
         }
     }
+    if(len<0)
+        goto error_redo;
     if(dts != AV_NOPTS_VALUE && ppos){
         int i;
         for(i=0; i<s->nb_streams; i++){