BuildHuffmanTable: add an assert for offset[] bounds

And provide a clear comment explaining why the index of offset[] is
always checked within bounds.

Bug:webp:622
Change-Id: Id9b973a804b74c53dfb291f1a9dae649c0daed9d

src/utils/huffman_utils.c

@@ -124,7 +124,11 @@ static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
     const int symbol_code_length = code_lengths[symbol];
     if (code_lengths[symbol] > 0) {
       if (sorted != NULL) {
-        if(offset[symbol_code_length] >= code_lengths_size) {
+        assert(offset[symbol_code_length] < code_lengths_size);
+        // The following check is not redundant with the assert. It prevents a
+        // potential buffer overflow that the optimizer might not be able to
+        // rule out on its own.
+        if (offset[symbol_code_length] >= code_lengths_size) {
           return 0;
         }
         sorted[offset[symbol_code_length]++] = symbol;