ralf: read Huffman code lengths without GetBitContext

Those descriptions are stored in nibbles, so they are easy to extract.
And this way we don't need to pad tables for possible bit reader overreads.

libavcodec/ralf.c

@@ -80,17 +80,17 @@ static int init_ralf_vlc(VLC *vlc, const uint8_t *data, int elems)
     int counts[17], prefixes[18];
     int i, cur_len;
     int max_bits = 0;
-    GetBitContext gb;
-
-    init_get_bits(&gb, data, elems * 4);
+    int nb = 0;
 
     for (i = 0; i <= 16; i++)
         counts[i] = 0;
     for (i = 0; i < elems; i++) {
-        cur_len  = get_bits(&gb, 4) + 1;
+        cur_len  = (nb ? *data & 0xF : *data >> 4) + 1;
         counts[cur_len]++;
         max_bits = FFMAX(max_bits, cur_len);
         lens[i]  = cur_len;
+        data    += nb;
+        nb      ^= 1;
     }
     prefixes[1] = 0;
     for (i = 1; i <= 16; i++)