webp: add a special case for a huffman table with only 1 symbol

The vlc reader cannot handle 0-bit huffman codes. For most
situations WebP uses the "simple" huffman coding for this case,
but that will only handle symbols up to 255. For the LZ77 distance
codes, larger symbol values are needed, so it can happen in rare
cases that a normal huffman table is used that only has a single
symbol.

libavcodec/webp.c

@@ -277,10 +277,26 @@ static int huff_reader_get_symbol(HuffReader *r, GetBitContext *gb)
 static int huff_reader_build_canonical(HuffReader *r, int *code_lengths,
                                        int alphabet_size)
 {
-    int len, sym, code, ret;
+    int len = 0, sym, code = 0, ret;
     int max_code_length = 0;
     uint16_t *codes;
 
+    /* special-case 1 symbol since the vlc reader cannot handle it */
+    for (sym = 0; sym < alphabet_size; sym++) {
+        if (code_lengths[sym] > 0) {
+            len++;
+            code = sym;
+            if (len > 1)
+                break;
+        }
+    }
+    if (len == 1) {
+        r->nb_symbols = 1;
+        r->simple_symbols[0] = code;
+        r->simple = 1;
+        return 0;
+    }
+
     for (sym = 0; sym < alphabet_size; sym++)
         max_code_length = FFMAX(max_code_length, code_lengths[sym]);