Avoid negative shifts in build_table()

A shift by a negative amount has undefined behaviour.  Even though
the result of this shift is never used, the shift itself could
cause an exception of some kind.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@21939 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/bitstream.c

@@ -158,11 +158,12 @@ static int build_table(VLC *vlc, int table_nb_bits,
 #endif
         /* if code matches the prefix, it is in the table */
         n -= n_prefix;
+        if (n > 0) {
         if(flags & INIT_VLC_LE)
             code_prefix2= code & (n_prefix>=32 ? 0xffffffff : (1 << n_prefix)-1);
         else
             code_prefix2= code >> n;
-        if (n > 0 && code_prefix2 == code_prefix) {
+        if (code_prefix2 == code_prefix) {
             if (n <= table_nb_bits) {
                 /* no need to add another table */
                 j = (code << (table_nb_bits - n)) & (table_size - 1);
@@ -197,6 +198,7 @@ static int build_table(VLC *vlc, int table_nb_bits,
             }
         }
         }
+    }
 
     /* second pass : fill auxillary tables recursively */
     for(i=0;i<table_size;i++) {