Fix assert bug in base58 encode

The previous implementation was almost certainly a typo.
full_block_size is the maximum index in the encoded_block_sizes array,
and size is used as an index in this array.  So now 1 <= size <=
full_block_size == 8 instead of 1 <= size <= sizeof(full_block_size) ==
size_of(size_t) == ? (maybe 4 on 32-bit systems!)

src/common/base58.cpp

@@ -110,7 +110,7 @@ namespace tools
 
       void encode_block(const char* block, size_t size, char* res)
       {
-        assert(1 <= size && size <= sizeof(full_block_size));
+        assert(1 <= size && size <= full_block_size);
 
         uint64_t num = uint_8be_to_64(reinterpret_cast<const uint8_t*>(block), size);
         int i = static_cast<int>(encoded_block_sizes[size]) - 1;