Fix possible crashes in mlp parser, it tries to go back 7 bytes after

finding the 4-byte signature.
Add a check that ignores the signature if we do not have enough previous
data to go back at least 7 bytes.

Originally committed as revision 21487 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mlp_parser.c

@@ -176,7 +176,9 @@ static int mlp_parse(AVCodecParserContext *s,
 
         for (i = 0; i < buf_size; i++) {
             mp->pc.state = (mp->pc.state << 8) | buf[i];
-            if ((mp->pc.state & 0xfffffffe) == 0xf8726fba) {
+            if ((mp->pc.state & 0xfffffffe) == 0xf8726fba &&
+                // ignore if we do not have the data for the start of header
+                mp->pc.index + i >= 7) {
                 mp->in_sync = 1;
                 mp->bytes_left = 0;
                 break;