mjpegbdec: Fix overflow in SOS.

Based in part by a fix from Michael Niedermayer <michaelni@gmx.at>

Fixes CVE-2011-3947

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

libavcodec/mjpegbdec.c

@@ -59,6 +59,9 @@ read_header:
     s->restart_count = 0;
     s->mjpb_skiptosod = 0;
 
+    if (buf_end - buf_ptr >= 1 << 28)
+        return AVERROR_INVALIDDATA;
+
     init_get_bits(&hgb, buf_ptr, /*buf_size*/(buf_end - buf_ptr)*8);
 
     skip_bits(&hgb, 32); /* reserved zeros */
@@ -111,8 +114,8 @@ read_header:
     av_log(avctx, AV_LOG_DEBUG, "sod offs: 0x%x\n", sod_offs);
     if (sos_offs)
     {
-//        init_get_bits(&s->gb, buf+sos_offs, (buf_end - (buf+sos_offs))*8);
-        init_get_bits(&s->gb, buf_ptr+sos_offs, field_size*8);
+        init_get_bits(&s->gb, buf_ptr + sos_offs,
+                      8 * FFMIN(field_size, buf_end - buf_ptr - sos_offs));
         s->mjpb_skiptosod = (sod_offs - sos_offs - show_bits(&s->gb, 16));
         s->start_code = SOS;
         if (ff_mjpeg_decode_sos(s, NULL, NULL) < 0 &&