musepack: fix signed shift overflow in mpc_read_packet()

Using an unsigned variable avoids problems with overflows.
There is further no need for a 64-bit intermediate here.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/mpc.c

@@ -117,7 +117,8 @@ static int mpc_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
     MPCContext *c = s->priv_data;
     int ret, size, size2, curbits, cur = c->curframe;
-    int64_t tmp, pos;
+    unsigned tmp;
+    int64_t pos;
 
     if (c->curframe >= c->fcount && c->fcount)
         return -1;
@@ -134,8 +135,7 @@ static int mpc_read_packet(AVFormatContext *s, AVPacket *pkt)
     if(curbits <= 12){
         size2 = (tmp >> (12 - curbits)) & 0xFFFFF;
     }else{
-        tmp = (tmp << 32) | avio_rl32(s->pb);
-        size2 = (tmp >> (44 - curbits)) & 0xFFFFF;
+        size2 = (tmp << (curbits - 12) | avio_rl32(s->pb) >> (44 - curbits)) & 0xFFFFF;
     }
     curbits += 20;
     avio_seek(s->pb, pos, SEEK_SET);