archive/iso9660: fix off-by-one assertion failure

Calling data[fill] could trigger an assertion failure if
fill==data.size(), even if we call it only to take the address.

Instead of doing that, this commit changes the code to pointer
arithmetic.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1556

src/archive/plugins/Iso9660ArchivePlugin.cxx

@@ -166,7 +166,7 @@ class Iso9660InputStream final : public InputStream {
 			assert(fill <= data.size());
 			assert(position <= fill);
 
-			return {&data[position], &data[fill]};
+			return {data.data() + position, data.data() + fill};
 		}
 
 		void Consume(size_t nbytes) noexcept {