Fix undefined shift on assumed 8-bit input.

decode_user_data() attempts to create an integer |build|
value with 8 bits of spacing for 3 components. However
each component is an int32_t, so shifting each component
is undefined for values outside of the 8 bit range.

This patch simply clamps input to 8-bits per component
and prints out a warning that the values were clamped.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -2153,8 +2153,15 @@ static int decode_user_data(Mpeg4DecContext *ctx, GetBitContext *gb)
         e = sscanf(buf, "FFmpeg v%d.%d.%d / libavcodec build: %d", &ver, &ver2, &ver3, &build);
     if (e != 4) {
         e = sscanf(buf, "Lavc%d.%d.%d", &ver, &ver2, &ver3) + 1;
-        if (e > 1)
-            build = (ver << 16) + (ver2 << 8) + ver3;
+        if (e > 1) {
+            if (ver > 0xFF || ver2 > 0xFF || ver3 > 0xFF) {
+                av_log(s->avctx, AV_LOG_WARNING,
+                     "Unknown Lavc version string encountered, %d.%d.%d; "
+                     "clamping sub-version values to 8-bits.\n",
+                     ver, ver2, ver3);
+            }
+            build = ((ver & 0xFF) << 16) + ((ver2 & 0xFF) << 8) + (ver3 & 0xFF);
+        }
     }
     if (e != 4) {
         if (strcmp(buf, "ffmpeg") == 0)